Depois do almoço, a minha barriga dói um pouco, e o meu pescoço fica tenso quando estou cansado.

Because do is the contraction of de + o.

• depois de = after
• o almoço = the lunch / lunch

So:

• depois de o almoço → depois do almoço

Portuguese very often contracts de with the definite article:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

Yes. In European Portuguese, depois de almoço is very common and natural.

There is a small nuance:

• depois do almoço often sounds a bit more like after the lunch / after lunch
• depois de almoço is a very common meal-time expression in Portugal, more like after lunch in a general everyday sense

Both are correct, and both are heard in Portugal.

In European Portuguese, it is standard and very common to use the definite article before possessives:

• a minha barriga
• o meu pescoço
• o meu carro
• a minha casa

So for Portugal Portuguese, a minha and o meu sound very normal.

Leaving out the article is much less common in standard European Portuguese, although you may see or hear it in special styles, fixed expressions, or in other varieties of Portuguese.

Barriga literally means belly, tummy, or the stomach area in an everyday sense.

It is less anatomical than estômago.

• barriga = belly/tummy area, everyday word
• estômago = stomach, more anatomical or medical

So:

• A minha barriga dói = My stomach/tummy hurts
• O meu estômago dói = My stomach hurts

In everyday speech, barriga is often the more natural choice when talking about general discomfort after eating.

Because in Portuguese, the thing that hurts is often the grammatical subject.

Here, the subject is a minha barriga, not eu.

So the verb agrees with barriga, which is third person singular:

• a minha barriga dói = my stomach hurts

This is different from English, where learners may expect something built around I.

A very common Portuguese pattern is:

• A cabeça dói
• O braço dói
• As costas doem

Notice the agreement:

• singular noun → dói
• plural noun → doem

Yes, absolutely. In fact, that is very natural in European Portuguese.

Both are correct:

• A minha barriga dói
• Dói-me a barriga

The second structure is very common in Portugal. It literally works like:

• dói-me = hurts me
• a barriga = the stomach/belly

So:

• Dói-me a barriga = My stomach hurts

This is a useful pattern in European Portuguese:

• Dói-me a cabeça
• Doem-me os pés
• Dói-me o pescoço

Dói is the present indicative, third person singular, of the verb doer.

So:

• doer = to hurt
• dói = hurts

Examples:

• A barriga dói = The stomach hurts
• O braço dói = The arm hurts
• A cabeça dói = The head hurts

The accent in dói shows the stressed sound and helps distinguish the pronunciation.

Um pouco means a little or a bit.

In this sentence, it modifies the idea of pain:

• a minha barriga dói um pouco = my stomach hurts a little

Putting um pouco after the verb is very natural.

You may also hear similar expressions such as:

• dói um bocadinho = it hurts a little bit
• dói só um pouco = it only hurts a little

So the position in the sentence is normal and idiomatic.

Because ficar + adjective often means to become, to get, or to end up in a state.

So:

• o meu pescoço fica tenso = my neck gets tense

This suggests a change of state, especially as a result of being tired.

Compare:

• fica tenso = gets tense / becomes tense
• está tenso = is tense

So in this sentence, fica tenso is a very good choice because it expresses what happens when the speaker is tired.

Because tenso agrees with o meu pescoço, which is masculine singular.

• o pescoço → masculine singular
• therefore tenso

If the noun were feminine, the adjective would change:

• A nuca fica tensa

So here the adjective agrees with pescoço, not with the speaker.

Here, cansado agrees with eu, the speaker.

So:

• male speaker: estou cansado
• female speaker: estou cansada

This is different from tenso in o meu pescoço fica tenso.

• tenso agrees with pescoço
• cansado/cansada agrees with the person speaking

So if a woman says the full sentence, she would normally say:

• Depois do almoço, a minha barriga dói um pouco, e o meu pescoço fica tenso quando estou cansada.

Because the sentence describes a habitual or typical situation, not just one single event.

It means something like:

• this usually happens after lunch
• this is what happens when I am tired

Portuguese uses the present tense for this kind of general, repeated fact:

• dói
• fica
• estou

So the sentence sounds like a description of the speaker’s usual experience.

The comma after Depois do almoço is very natural, because that phrase is an introductory time expression.

So this is standard:

• Depois do almoço, ...

The comma before e is not always strictly necessary, but it is acceptable and helpful here because the sentence joins two fairly long clauses with different subjects:

• a minha barriga
• o meu pescoço

So the punctuation in the sentence is perfectly normal.

Yes, there are a few very natural European Portuguese alternatives, especially for the first part.

For example:

• Depois de almoço, dói-me um pouco a barriga, e o meu pescoço fica tenso quando estou cansado.
• Depois de almoço, fico com um pouco de dor de barriga, e o meu pescoço fica tenso quando estou cansado.

The original sentence is correct and understandable, but learners should know that European Portuguese often likes structures such as:

• dói-me a barriga
• dói-me a cabeça
• doem-me as costas

So the given sentence is good Portuguese, and there are also slightly more idiomatic Portugal-style alternatives.