Weight loss after a baby

Is there any REAL or scientific reason behind it being harder to lose weight after a baby? All my life I’ve been incredibly fit and toned, I gained about 50lbs pounds during my pregnancy, 10lbs came off when I left the hospital and I was incredibly confident it wouldn’t take me longer than 4 months if that, to lose the rest. Before my son if I wanted to lose a quick 15lbs, give me 2-3 weeks and it would happen. I’m now 13 months pp and I’m at the gym 5 days a week and I eat clean and have hardly scratched the surface. Its seriously frustrating. So is it real? Or am I just doing something wrong, at this point I want to give up.