Ah. Don't you LOVE it. Just when you think this is great, they sleep through the night-they throw a curve ball at you. K is 3 1/2. This is a prime age for nightmares. And at this age they don't necessarily understand that they aren't real. Start asking that. You could ask when you go in, but K would never tell me then. So I'd ask in the morning. He usually didn't want to talk about it-who could blame him? Asking him to relive something he viewed as real and scary. But we could usually find out some details. Then you can go from there on how to help her.
But, waking up at certain is a habit. Which they can form very easily. And if she is waking up at the same time pretty much each night, then that is what you are probably dealing with. Is she asking for anything when you go in? Water, cover me, etc.? If it is something that you can solve before it happens-DO IT! K started waking up asking for water. Went on for about a week, then I started putting a sippy w/water next to his bed. Recently it has been wanting ME to re-cover him up. Something he CAN do. But why do it when I can wake Mama up in the middle of the night? So I started making sure the blanket was tucked in at the bottom so it couldn't come loose. And before I go to bed I re-cover him. My husband makes sure he is covered before he goes to bed and if I wake up in the middle of the night I check then too.
If there is no reason other than she just wants you to sit there, can you call out and let her know that you are right there, that you are sleeping and you love her will see her in the morning? Go in and tell her you love her but you are tired and need to go back to sleep? Does she have any comfort items? We have a music thing-FP thing for cribs-that K turns on. Breaking the habit is HARD and the longer it goes on it will only get HARDER.
I hope that you can figure out what is going on and can get back to sleeping through the night.