Se eu dormir pouco de novo, vou ficar com dor na cabeça.

Because Portuguese normally uses the future subjunctive after se when talking about a real future possibility:

Se eu dormir pouco de novo, vou ficar...
= If I sleep too little again, I’ll...

So:

• se eu dormir = if I sleep
• se eu durmo is not the normal form here
• se eu dormisse would mean something more hypothetical or less likely: if I were to sleep / if I slept

Compare:

• Se eu dormir pouco, vou ficar cansado. = If I sleep little, I’ll get tired.
• Se eu dormisse pouco, ficaria cansado. = If I slept little, I would get tired.

One useful detail: for many verbs, the future subjunctive form looks the same as the infinitive, so dormir here is not the plain infinitive in function, even though it looks identical.

Here pouco means little / not much / not enough.

So dormir pouco means:

• to sleep little
• to not get much sleep
• to not sleep enough

In this sentence, it suggests a lack of sleep, not just a short nap.

Because the two expressions mean different things:

• dormir pouco = to sleep too little / not sleep much
• dormir um pouco = to sleep a little / sleep for a bit

That difference matters a lot here.

Examples:

• Eu dormi pouco. = I didn’t sleep much.
• Eu dormi um pouco. = I slept a little.

So in your sentence, pouco is about insufficient sleep, which matches the idea of getting a headache afterward.

De novo means again.

So dormir pouco de novo means to sleep too little again.

It tells you this is happening another time, not for the first time.

You could also hear:

• outra vez
• novamente

But de novo is extremely common in everyday Brazilian Portuguese.

Vou ficar is the very common spoken way to express the future in Brazilian Portuguese.

So:

• vou ficar = I’m going to get / I’ll end up
• ficarei = I will get

Both are correct, but vou ficar sounds more natural in everyday speech.

Also, ficar here has the idea of entering a state or ending up in a condition. It is not just to be. It is more like:

• to become
• to get
• to end up

So vou ficar com dor... is more like I’m going to get pain / end up with pain.

Here ficar com means to get or to end up with.

So:

• ficar com dor = to get pain / end up with pain
• ficar com febre = to get a fever
• ficar com sono = to get sleepy

In your sentence:

vou ficar com dor na cabeça
= I’m going to get pain in my head / I’m going to end up with a headache

This is a very common pattern in Portuguese.

It is understandable, but dor de cabeça is usually the more standard and idiomatic way to say headache.

So many native speakers would more naturally say:

Se eu dormir pouco de novo, vou ficar com dor de cabeça.

Difference:

• dor de cabeça = headache
• dor na cabeça = pain in the head

The second one is more literal and can sound like you are emphasizing the location of the pain. It is not wrong, but dor de cabeça is the expression learners will hear most often.

Na is a contraction of:

• em
  • a = na

So:

• na cabeça = in/on the head

This happens all the time in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

So dor na cabeça literally has the structure pain in the head.

Because Portuguese often uses the definite article with body parts when it is obvious whose body part it is.

So native speakers often say:

• Estou com dor na cabeça.
• Lavei as mãos.
• Machuquei o braço.

Instead of always saying:

• na minha cabeça
• minhas mãos
• meu braço

Adding minha is possible, but it usually adds emphasis, contrast, or extra specificity.

So:

• dor na cabeça = neutral, natural
• dor na minha cabeça = more emphatic or more explicit than necessary in most contexts

Because the sentence starts with a conditional clause:

Se eu dormir pouco de novo, ...

That introductory if-clause is commonly followed by a comma in Portuguese.

So this is the standard punctuation:

Se eu dormir pouco de novo, vou ficar com dor na cabeça.

You could also reverse the order:

Vou ficar com dor na cabeça se eu dormir pouco de novo.

In that order, the comma is usually not needed.

Yes. That would sound very natural.

Compare:

• vou ficar com dor de cabeça = I’m going to get a headache
• vou ter dor de cabeça = I’m going to have a headache

Both work well.
The version with ficar com can feel a little more like ending up with that condition, while ter is a straightforward have.

For everyday Brazilian Portuguese, both are common and natural.