First day back at work

And my body is killing me.. feels like when I first gave birth and I was recovering.. couldn’t lift my legs or anything and I literally can’t do that even now.. I work at Walmart so I’m walking a lot and carrying heavy things.. why do I feel like I gave birth all over again? Like my legs, thighs and back is throbbing and my feet ache horribly.. I took 9 weeks off for maternity leave so maybe I’m overdoing it coming from not doing to much for a while?