Me duele la cintura después de estar tantas horas sentada.

Because doler works like gustar.

• Me duele la cintura literally works like The waist hurts to me
• In natural English, we say My waist hurts
• In Spanish, the thing that hurts is the grammatical subject, and the person affected is shown with an indirect object pronoun: me, te, le, nos, os, les

So:

• me duele la cintura = my waist hurts
• te duele la cabeza = your head hurts
• me duelen las piernas = my legs hurt

Yo duele is incorrect here.
If you said yo duelo, that would mean I hurt / I cause pain, which is a different idea.

Because la cintura is singular.

With doler, the verb agrees with the thing that hurts:

• Me duele la cintura → one thing hurts
• Me duelen las piernas → more than one thing hurts

So the pattern is:

• singular noun → duele
• plural noun → duelen

Spanish usually uses the definite article with body parts when it is already clear whose body part it is.

Here, me already tells you whose waist it is, so la cintura sounds natural.

• Me duele la cintura = literally The waist hurts me
• Natural English translation: My waist hurts

You can say mi cintura, but it is less neutral and often sounds more emphatic, contrastive, or stylistically marked.

The basic meaning of cintura is waist.

In health or pain contexts, though, speakers may use it to refer to the waist/lower-back area. So depending on context, it may sound broader than just the narrow anatomical waist.

If you want to be more specific, you could also hear:

• la espalda = the back
• la parte baja de la espalda = the lower back
• la zona lumbar = the lumbar area / lower back

So cintura here is best understood as the area around the waist, possibly including the lower back.

Because after a preposition like de, Spanish normally uses the infinitive when the subject is the same.

So:

• después de estar tantas horas sentada = after being / after spending so many hours sitting

You cannot say:

• después de estoy...
• después de estaba...

Those are ungrammatical after de.

This structure is very common:

• después de comer = after eating
• después de trabajar = after working
• después de estar aquí = after being here

Because sentada agrees with the speaker’s gender.

With estar + adjective/past participle used adjectivally, the ending changes:

• sentado = masculine singular
• sentada = feminine singular
• sentados = masculine/mixed plural
• sentadas = feminine plural

So this sentence implies the speaker is female:

• Me duele la cintura después de estar tantas horas sentada.

A male speaker would say:

• Me duele la cintura después de estar tantas horas sentado.

Because estar sentada describes a state or position: being seated / sitting.

• estar sentada = to be sitting / to be seated
• sentarse = to sit down

This sentence talks about the result of remaining in a seated position for many hours, not the action of sitting down once.

Compare:

• Se sentó = she sat down
• Estuvo sentada muchas horas = she was sitting for many hours

So estar sentada is exactly the right choice here.

Tantas means so many or that many, and it agrees with horas.

Since horas is:

• feminine
• plural

the form must be tantas.

Related forms:

• tanto tiempo = so much time
• tantas horas = so many hours
• tantos días = so many days
• tantas veces = so many times

So tantas horas means so many hours.

Because Spanish often drops subject pronouns when they are not needed.

In this sentence, the speaker is already understood from me, so yo is unnecessary.

• Me duele la cintura... = normal
• Yo me duele la cintura... = wrong
• A mí me duele la cintura... = possible for emphasis or contrast

If you want emphasis, Spanish usually uses a mí rather than yo here:

• A mí me duele la cintura, pero a ella no.

Yes, but the meaning changes slightly.

• después de estar tantas horas sentada = after sitting for so many hours
  • focuses on what happened next in time
• por estar tantas horas sentada = because of sitting for so many hours
  • focuses more directly on the cause

Both can make sense, but they are not identical.

So:

• Me duele la cintura después de estar tantas horas sentada.
  = My waist hurts after sitting so many hours.

• Me duele la cintura por estar tantas horas sentada.
  = My waist hurts because of sitting so many hours.

Horas is enough here. Spanish often expresses duration directly with a time expression.

• estar tantas horas sentada = to be sitting for so many hours

You could also say:

• después de estar sentada durante tantas horas

That is grammatical, but usually less natural in everyday speech. The original version is simpler and very normal.

Yes, if you mean back rather than waist.

Compare:

• Me duele la cintura = my waist / waist area hurts
• Me duele la espalda = my back hurts

If the pain is specifically in the lower back, you might also hear:

• Me duele la parte baja de la espalda
• Me duele la zona lumbar

So the best word depends on the exact part of the body you mean.