Hi there,
my son did this also at around the same age. I think sometimes he would sit up before he actually woke and then cried becasue he wasn't sure how he had got there. We would just go in and lay him back down, whisper to him that he was ok and he could go back to sleep and generally he would. A few times we would need to pick him up, give a quick cuddle and put him back down.
Also, your daughter may be coming into a separation anxiety phase where she's discovering that she is a different person from you and may just need reassurance that you are there. Its ok to go to her and comfort her when she's upset, just make sure you don't comfort past her need or you may create a dependency that you don't want.
I'm sure all the other mummies will have some more advice but just want to let you know that you're not alone and it will pass in time!!
Keep us updated on your progress.