I agree with the mom that said she should still be in a crib, in stead of a bed.
Also, it's possible that her diaper is wet and uncomfortable, make sure that you don't let her have anything else to drink after her final change for the night.
Our 2 year old started having trouble after she was sleeping in a bed, instead of in a crib, but we found that it was because she was wet. When we solved that problem, she started sleeping much better. She is almost 2.5 years old, and is the size of a 3.5 year old, the crib is no longer an option for her, though I kept her in it as long as I could. She always slept best in there.