Después de tantas horas sentada, me dolía la cadera y tuve que levantarme.

Both are possible, but they are structured differently.

• Después de tantas horas sentada = after so many hours sitting / after so many hours being seated
• Después de estar sentada tantas horas = after being seated for so many hours

In the original sentence, después de + noun phrase is being used:

• después de tantas horas = after so many hours

Then sentada describes the speaker’s state during those hours.

This is a very natural, compact way to say it in Spanish.

Because sentada agrees with the speaker, who is understood to be female.

• sentado = masculine singular
• sentada = feminine singular

So:

• If a man says it: Después de tantas horas sentado...
• If a woman says it: Después de tantas horas sentada...

Spanish adjectives and participle-like forms often agree in gender and number with the person they describe.

Here, sentada is functioning like an adjective/state description, not as part of a tense.

It comes from sentar / sentarse, but in this sentence it means:

• seated
• sitting

So tantas horas sentada means so many hours seated/sitting.

This is similar to how Spanish often uses a past participle or adjective to describe someone’s state:

• estaba cansada = she was tired
• pasé el día preocupada = I spent the day worried

Tantas means so many and it must agree with horas, which is feminine plural.

• tanto tiempo = so much time
• tantas horas = so many hours

Agreement:

• tanto
  • masculine singular noun
• tanta
  • feminine singular noun
• tantos
  • masculine plural noun
• tantas
  • feminine plural noun

Because horas is feminine plural, the sentence uses tantas.

Spanish commonly uses an indirect object pronoun with body parts:

• me dolía la cadera = my hip hurt literally: the hip was hurting to me

This is more natural in Spanish than using a possessive like mi.

The usual pattern is:

• me duele la cabeza
• te duelen los pies
• le duele el brazo

You can say mi cadera, but it is less natural unless you want to emphasize my hip specifically.

Dolía is the imperfect, which is used for an ongoing or background situation in the past.

Here it suggests:

• the pain was continuing
• it was not just a single sudden moment

So:

• me dolía la cadera = my hip was hurting / my hip hurt

If you said me dolió, it would sound more like:

• it hurt at a specific moment
• the pain is viewed as a completed event

In this sentence, the ongoing pain forms the background, and then the action happens:

• me dolía la cadera = ongoing situation
• tuve que levantarme = resulting action

Because the speaker is talking about one hip, not both.

• la cadera = the hip
• las caderas = the hips

If both hips hurt, you could say:

• me dolían las caderas

Notice that the verb would also become plural:

• dolían because las caderas is plural

Both can mean had to, but they are used differently.

• tuve que = I had to, as a specific completed event
• tenía que = I had to / I was supposed to / I needed to, more as an ongoing situation or obligation

In the sentence:

• me dolía la cadera y tuve que levantarme

This means the pain reached the point where the speaker ended up having to get up. It is a completed action, so tuve que fits very well.

Compare:

• Tenía que levantarme cada hora = I had to get up every hour
• Tuve que levantarme = I had to get up

Because levantarse means to get up / stand up, and it is a reflexive verb.

• levantar = to lift / raise something
• levantarse = to get up

So:

• tuve que levantarme = I had to get up

The me refers back to the speaker.

Examples:

• Voy a levantar la caja = I’m going to lift the box
• Voy a levantarme = I’m going to get up

Yes, both are correct:

• tuve que levantarme
• me tuve que levantar

When you have tener que + infinitive with a reflexive verb, the reflexive pronoun can go:

1. attached to the infinitive

   • tuve que levantarme

2. before the conjugated verb

   • me tuve que levantar

The first version is very common and often sounds slightly smoother here.

A comma is often optional here depending on style.

You may see:

• Después de tantas horas sentada, me dolía la cadera...
• Después de tantas horas sentada me dolía la cadera...

The comma helps mark the introductory phrase:

• Después de tantas horas sentada

So using a comma is very normal and often clearer, but not having one would not necessarily be wrong in all contexts.

No. Cadera means hip, not waist.

• cadera = hip
• cintura = waist

This is an important vocabulary distinction for English speakers.

Examples:

• Me duele la cadera = My hip hurts
• Tengo una cintura estrecha = I have a narrow waist

Normally, no. Sentándome means sitting down in the sense of the action of taking a seat, not remaining seated.

So:

• sentarse can mean to sit down
• estar sentado/a or quedarse sentado/a expresses being in a seated position

The sentence is about the result of having been seated for a long time, so sentada is the natural choice.

Using sentándome would suggest repeated or ongoing movement of sitting down, which is not the intended meaning here.