It could be postural. I went to the GP and was fobbed off with cocodamol and told to be patient with my PP recovery as everyone's different. I paid for a Mummy MOT and have found that it's due to postural changes due to extra weight in pregnancy and a weak part of my back then my other side and joints compensating so got some exercises to do and I feel amazing ever since!
I was like this with my first baby and I had an undiagnosed under active thyroid. I also had other symptoms as well that I just passed off as post partum not knowing it wasn't normal been a first time mum 🙈 Not sure if that might be worth speaking to your GP about x