Yes I agree, take him/her to a vet. Just because a hamster is small doesnt mean they don't feel pain. It would be cruel just to leave it in the cage and hope it gets better. Hamsters are very fragile. You should keep it away from cats, as they are natural enemies.
Our hamster was attacked by our kitten and the result was a severe wound on the hamster's back. My friend is a vet tech and she came over and cleaned the wound with a saline solution and applied Neosporin to use as an antibiotic to prevent the wound from becoming infected. The wound healed in less than three weeks and he appears to be fine. She cautioned me, however, to keep him in a very stable environment, meaning no lights on and off all the time, no loud noises (we move him out of the room if we vacuum) and by no means, any contact with the kitten. Apparently, they're more susceptible to the shock than to the injury. For the limping, we did not have that problem, but I'd see the vet for that one.
