My hamster has a swollen leg! :(?

well, what happened was; i came home from a week at my cottage and found my hamsters sleeping (this is obviously normal) but when i open their cages they wake up and run away or hide. my teddy-bear hamster (Sparta) did this, but not my black bear hamster (penny). she stood up and kinda waddled a little bit, then laid back down so i picked her up and one of her back legs has a weird sort of bubble-like swelling on it where the fur stops growing on her leg. i asked my step sister (who is 21 and was taking care of my hamsters) if anything had happened and apparently she had escaped but not jumped off her cage, and was just found standing on top of it. she has escaped before but i dont know if the swelling is a break or if its a tumor or something. should i do something about it? what is it?