geunyeoneun jusareul matgo nanikka pari jogeum apasseo.

That is a very common point of confusion.

In Korean, 주사를 맞다 is the standard expression for to get/receive a shot or to have an injection. It uses the verb 맞다, which in other contexts can mean to be hit or to be struck.

The idea is that something is physically coming into contact with your body, so Korean uses 맞다 in several expressions like this.

For example:

• 주사를 맞다 = to get an injection
• 예방접종을 맞다 = to get a vaccination
• 총을 맞다 = to be shot
• 비를 맞다 = to get caught in the rain / be rained on

So in this sentence, 주사를 맞고 나니까 means after getting a shot.

-고 나니까 means after doing X, once X happened, or when/after I did X, I found that...

So:

• 맞고 나니까 = after getting the injection / once she got the injection

This pattern often introduces something that became clear afterward, especially a result, change, or realization.

In this sentence:

• 주사를 맞고 나니까 팔이 조금 아팠어
• After she got the shot, her arm hurt a little

So -고 나니까 is not just a neutral sequence marker. It often suggests:

1. first one action happened,
2. then a result or realization followed.

They are similar, but not identical.

• -고 나서 = simply after doing
• -고 나니까 = after doing, it turned out that... / once I did it, then...

So -고 나니까 often has a stronger sense of a following result, discovery, or consequence.

Compare:

• 주사를 맞고 나서 집에 갔어.
  After getting the shot, she went home.
  -> just sequence

• 주사를 맞고 나니까 팔이 아팠어.
  After getting the shot, her arm hurt.
  -> result noticed after the action

In your sentence, -고 나니까 works well because the arm pain is something that happened or was noticed after the injection.

Because 주사 is the object of the verb 맞다 in this expression.

• 주사 = injection
• 주사를 맞다 = to get an injection

Here, -를 / -을 marks what is being received or undergone.

So:

• 주사를 맞다 = get a shot
• 예방접종을 맞다 = get a vaccination

Even though English says get a shot, Korean structures it as shot + object marker + 맞다.

This is about topic versus subject.

• 그녀는 marks she as the topic of the whole sentence.
• 팔이 marks arm as the subject of the predicate 아팠어.

In Korean, when talking about body conditions, the body part often becomes the grammatical subject:

• 머리가 아파요 = my head hurts / I have a headache
• 배가 아파요 = my stomach hurts
• 팔이 아팠어 = the arm hurt

So the sentence is structured roughly like:

• As for her, after getting the shot, her arm hurt a little.

If you said 팔은, that would add contrast or special topic emphasis, like:

• As for her arm, it hurt a little... That is possible in the right context, but 팔이 is the most natural neutral choice here.

Because 아프다 does not take the painful body part as a direct object in Korean.

In English, we often say:

• My arm hurt

In Korean, the body part is usually marked as the subject:

• 팔이 아프다 = the arm hurts

So:

• 팔이 조금 아팠어 = the arm hurt a little

Using 팔을 아팠어 would be ungrammatical in standard Korean.

This pattern is very common with body pain:

• 다리가 아파요 = my leg hurts
• 눈이 아파요 = my eyes hurt
• 목이 아파요 = my throat hurts

조금 means a little or slightly.

So:

• 팔이 조금 아팠어 = her arm hurt a little

It softens the statement and shows the pain was mild.

A few similar words:

• 조금 = a little
• 좀 = shortened conversational form of 조금
• 약간 = slightly, somewhat

In everyday speech, many people would also say:

• 팔이 좀 아팠어

That sounds very natural too.

아팠어 is the casual/informal past tense of 아프다.

Breakdown:

• dictionary form: 아프다 = to hurt, to be painful
• past stem: 아팠-
• casual ending: -어
• 아팠어 = hurt / was painful

This is a plain conversational style used with:

• friends
• younger people
• close relationships
• narration in an informal tone

Other style levels:

• 아팠어요 = polite
• 아팠다 = plain written/narrative style
• 아팠습니다 = formal polite

So the sentence sounds like casual spoken Korean.

Usually, not very natural in ordinary conversation.

In real spoken Korean, people often:

• omit the subject if it is clear from context, or
• use the person’s name, title, or relationship term instead

So instead of 그녀는, Korean speakers would often say:

• just 주사를 맞고 나니까 팔이 조금 아팠어
• or use a name, like 민지는 주사를 맞고 나니까...

그녀 does exist, but it often sounds:

• literary
• written
• translated
• somewhat unnatural in casual speech

So grammatically it is fine, but stylistically it may feel like written Korean more than everyday conversation.

It can do both.

In this sentence, 맞고 나니까 primarily means after getting the shot, but it also strongly suggests that the arm pain happened as a result of that event.

So the nuance is close to:

• After she got the shot, her arm hurt a little
• Once she got the shot, her arm started to hurt a little

It is not exactly the same as a direct cause marker like -아서/어서, but it often implies:

1. first event happened,
2. then a resulting condition was noticed.

That is why -고 나니까 fits very naturally here.

Yes, very naturally.

Korean often drops subjects and topics when they are understood from context. So if everyone already knows who you are talking about, you can simply say:

• 주사를 맞고 나니까 팔이 조금 아팠어.

That would usually sound more natural in conversation than explicitly saying 그녀는.

Korean frequently leaves out:

• I
• you
• he/she
• other obvious topics

So while 그녀는 is grammatically fine, it is often unnecessary.

Yes. 아팠어 is the simplest and most natural way to describe the resulting pain.

• 팔이 아팠어 = her arm hurt / was painful
• 팔이 아파졌어 = her arm became painful

The second one is grammatically possible, but it sounds more marked and emphasizes the change of state. In everyday Korean, people usually just say 아팠어 unless they specifically want to stress it became painful.

So in this sentence, 아팠어 is the most natural choice.