Probably the first time she pull on the connective tissue in that area post her surgery. This might have caused muscles such as her piriformis (A muscle in the butt area) to fire in an abnormal way. This is a common issue whether one has had a hip replacement or not. If she gets a physio to take a look at the pelvis and gluteal region they can probably send her on her way back to where she was a little faster. Sounds like she is already on the mend