Why is she screaming during her diaper change?

Recently (within the past few weeks) my 16 month old has started violently screaming and crying during her diaper change. Normally she never had a problem, she would actually get down on the floor, roll on her back and lift her legs up. Now when either myself or daycare provider tries to change her diaper her left leg stiffens straight up in the air to the point the her hip and leg muscles are extremely tight and she screams bloody murder and cries to the point that her face is soaked. This isn't her tantrum cry this is pain cry. I've also noticed she becomes the same way when she is sitting in her car seat or high chair, not as much as during the diaper changes though. I'm worried that something might be wrong with her hips, leg or back....What do you think this could be?