Depois do treinamento, estou com dor na cintura.

Because depois normally takes the preposition de when it is followed by a noun.

So:

• depois de = after
• o treinamento = the training session / the workout / the training

When de + o come together, they contract:

• de + o = do

So:

• depois do treinamento = after the training

This kind of contraction is very common in Portuguese.

They are close, but not always identical.

• treinamento often sounds a bit more formal or structured, like training
• treino is very common in everyday Brazilian Portuguese, especially for gym or sports, like workout or training session

In this sentence, both could work depending on context:

• Depois do treinamento...
• Depois do treino...

In casual speech, many Brazilians would probably say depois do treino if they mean a gym workout.

In Portuguese, estar com dor is a very common way to say that someone is feeling pain.

Literally, it is like:

• estou = I am
• com dor = with pain

So estou com dor means I’m in pain or I have pain in natural English.

Portuguese often uses estar com for temporary physical states:

• estou com fome = I’m hungry
• estou com sono = I’m sleepy
• estou com dor = I’m in pain

It is a very natural structure.

You might hear it, but estou com dor na cintura sounds more natural for a temporary situation, especially right now.

Compare:

• estou com dor na cintura = I have pain in my waist/lower back right now
• tenho dor na cintura = I have waist/lower-back pain, possibly as a more general or recurring condition

So if the idea is after training, I’m feeling pain there, estou com dor is the better choice.

Because na is a contraction of:

• em + a = na

And cintura is a feminine noun:

• a cintura

So:

• dor na cintura = pain in the waist

This is another very common contraction in Portuguese, just like do.

Not always exactly.

Cintura usually refers to the waist area, but in everyday Brazilian Portuguese, dor na cintura can sometimes sound like pain around the waist/lower back/side-of-the-body area, depending on context.

So a learner should be careful: the Portuguese word is often broader in feeling than the English word waist.

If someone wants to be more specific, they might say:

• dor nas costas = back pain
• dor na lombar = lower-back pain
• dor no quadril = hip pain

So cintura is understandable, but the exact body area can depend on the situation.

Because in this expression, Portuguese normally just says com dor, not com a dor, when speaking generally about pain.

So:

• estou com dor = I’m in pain / I have pain

If you said com a dor, it would usually refer to a specific pain already mentioned before, which is not the case here.

This is similar to how many common expressions work in Portuguese: the noun appears without an article in a general sense.

Yes, you could.

• sinto dor na cintura = I feel pain in my waist/lower back

This is grammatical and clear, but it can sound a little more direct or slightly more formal depending on context.

In everyday speech, estou com dor na cintura is often more natural when talking about your current physical condition.

Yes, you can move it.

These are all possible:

• Depois do treinamento, estou com dor na cintura.
• Estou com dor na cintura depois do treinamento.
• Estou com dor na cintura depois do treino.

Putting depois do treinamento at the beginning helps set the scene first: After training...

That word order is very natural.

Because this is a temporary condition, not a permanent characteristic.

• ser is used for identity, definition, and more permanent qualities
• estar is used for temporary states and conditions

Pain after training is temporary, so Portuguese uses estar:

• estou com dor = I’m in pain

Using sou com dor would be incorrect.

Yes, it is understandable and natural, but many people might say a slightly more casual version:

• Depois do treino, estou com dor na cintura.

That said, your original sentence is perfectly normal. The main difference is that treino often sounds more conversational than treinamento in fitness or sports contexts.

Dor na cintura is the normal pattern here.

Portuguese often uses dor em + body part:

• dor na perna = pain in the leg
• dor no braço = pain in the arm
• dor na cintura = pain in the waist/lower-back area

There are also some fixed expressions with de, especially:

• dor de cabeça = headache
• dor de dente = toothache

So dor de cintura would sound unusual to most speakers. For this sentence, dor na cintura is the natural choice.

Yes.

Because Portuguese often uses the definite article where English may or may not use one, do treinamento can be translated naturally as:

• after training
• after the training
• after the training session

The best English translation depends on context, but the Portuguese phrase itself is completely normal.