Need help with my hamster?

A few hours ago I noticed that my black bear hamster. wasn't walking on his right hind foot, he puts weight on it a little bit but sometimes he just drags it his toes don't move either. His hip & his leg look swollen about the size of a golf ball. It's just on that side. I just got Rhino at PetCo 4 months ago. I really want to know what you think is wrong with him. Please help he's my baby I love him so much he's not even a year old yet. I don't want anything to happen to him :'{. I'm taking him to the vet either way. but I still want answers from you. Also he's been drinking water a lot and he's eating like he normally does.