same thing happened with me - its just from childbirth no other reason
I would get a second opinion to be safe. Some things especially arthritic conditions have a high rate of onset after child birth. Things like RA and AS
It could be from giving birth, but I would suggest she get a second opinion. I know after I gave birth, I had a lot of pelvic and hip pain the first year. It was pretty bad at times and made it hard to move because sometimes it would pop out of place (or so it seemed) and it was painful and I had to becareful how I moved. Thankfully it subsided, but happened again after my second child. Your body goes through a lot and shifts around to make room and deliver a baby.