Is it normal to have extremely week hind legs for a dog post bladder stone surgery?

My 4 yr old Daschund had a bladder stone lodged in his urethra so we opted to have it surgically removed. His recovery has been good until yesterday when he started dragging his hind legs. I am hoping he has exhausted himself and is extremely sore. He can move his hind legs and wag his tail but it is difficult to put weight and hold himself up and balance himself while walking. Is this normal please help me and give me some information!