2 year old not sleeping well. Help!

My toddler is 27 months and has always been an excellent sleeper. She was sleeping through the night by 12 weeks. She's never woken up unless she was sick. Well, for a couple months now, I'd say maybe 60-70% of the nights, she'll wake up in the middle of the night (usually once or twice) and cry for us, not like blood curdling screams, but just whines that she wants us. One of us gets up and puts her back to bed (she's in a twin bed and has been since 21 months). She normally does fine when we put her back and she'll fall asleep. I've been OK doing this, but now I'm just getting annoyed. She doesn't want anything, she just wakes up and wants us. She does have a speech delay (we're in therapy for it) so it's possible she just doesn't know how to say what she wants. But does anybody know why this keeps happening? Anyone have advice? We also just had her baby sister back in March. She was in the nicu for 2 weeks which was also when we transitioned my 2 year old to her bed. But she's been fine until June/July when these episodes have been happening. This morning I had to lay in bed with her until she fell back asleep. So any advice?