omoi nimotu wo motta ato, kosi ga itaku narimasita.

Because 重い is an い-adjective that directly modifies the noun 荷物.

So:

• 重い = heavy
• 荷物 = baggage / luggage / load
• 重い荷物 = heavy luggage / a heavy load

In Japanese, adjectives normally come before the noun they describe, just like in English.

を marks the direct object of the verb.

Here:

• 荷物 = the thing being carried
• 持った = carried / held

So 荷物を持った means carried the luggage or carried a heavy load.

A native English speaker may want to think of を as showing what received the action of the verb.

Because when あと means after doing something, the verb before it is often put in the plain past form.

So:

• 持つ = to carry
• 持った = carried
• 持ったあと = after carrying

This pattern is very common:

• 食べたあと = after eating
• 見たあと = after seeing
• 帰ったあと = after going home

Even though English might use after carrying, Japanese commonly uses the past form: carried + after.

Here, あと means after.

In this sentence, it connects two events:

1. carrying the heavy luggage
2. the back starting to hurt

So 重い荷物を持ったあと means after carrying heavy luggage.

A useful comparison:

• Vたあと = after doing V
• Vるまえ = before doing V

For example:

• 食べたあと = after eating
• 食べるまえ = before eating

Because 痛い describes a state of a body part, and the body part is usually marked with が.

So:

• 腰が痛い = my lower back hurts / the lower back is painful

Then:

• 痛くなりました = became painful / started to hurt

Together:

• 腰が痛くなりました = my lower back started to hurt

A common pattern in Japanese is:

• 頭が痛い = my head hurts
• お腹が痛い = my stomach hurts
• 喉が痛い = my throat hurts

English often uses my back hurts, but Japanese treats the body part more like the thing that is in a painful state, so が is natural.

Because なる attaches to the adverbial form of an い-adjective.

For い-adjectives, you change 〜い to 〜く before なる:

• 痛い → 痛くなる = to become painful
• 高い → 高くなる = to become expensive / high
• 早い → 早くなる = to become early / faster

So:

• 痛い = painful
• 痛くなる = become painful
• 痛くなりました = became painful / started to hurt

This is standard grammar, not a special irregular form.

The difference is state versus change.

• 腰が痛い = my lower back hurts / my lower back is painful
  → this describes the current condition

• 腰が痛くなりました = my lower back started to hurt / became painful
  → this emphasizes a change, that it was not painful before but then it became painful

In this sentence, the speaker is describing what happened after carrying something heavy, so 痛くなりました fits well.

なりました is the polite past form of なる.

• なった = became
• なりました = became / started to, in polite speech

The sentence is simply in a polite style. A plain-style version would be:

重い荷物を持ったあと、腰が痛くなった。

Both are grammatically correct. The difference is politeness and tone.

In this sentence, 腰 is best understood as lower back.

The word 腰 can cover a range of meanings depending on context, including:

• waist
• hips
• lower back

But with 痛い after carrying something heavy, the natural interpretation is lower back pain.

So 腰が痛くなりました is most naturally understood as my lower back started to hurt.

Because Japanese often omits information that is clear from context.

In English, we usually say:

• After I carried the heavy luggage, my lower back started to hurt.

In Japanese, the I is often unnecessary if the listener already understands who is being talked about.

So the sentence can naturally leave out 私は or 私が.

If you added it, you could say:

私は重い荷物を持ったあと、腰が痛くなりました。

But in many situations, that sounds less natural unless you need emphasis or contrast.

In isolation, 持つ can mean several things, including:

• hold
• carry
• have
• possess

But in this sentence, with heavy luggage, the natural meaning is carry.

So:

• 荷物を持つ can literally be hold luggage
• but here carry luggage is the best English choice

Context decides the most natural translation.

The comma after あと helps separate the time clause from the main statement.

So the structure is:

• 重い荷物を持ったあと、 = after carrying heavy luggage,
• 腰が痛くなりました。 = my lower back started to hurt.

Japanese commas are often used more flexibly than English commas, but here it makes the sentence easier to read by clearly marking the transition from the after... part to the main result.