hamster with open sore on stomach and lumps

Question
We got this hamster about a year ago. He's still eating and drinking. He has a large,open sore on his stomach and is losing fur on his top side. This happened quickly. I did change to the bedding that's like paper and this is when it happened,less then 5 days. The odor is unbearable.

Answer
what type of bedding did you use before and what is the brand that you use now? There are two possible things that could be going on, one is he is having an allergic reaction to the new bedding or he could have a serious skin reaction going on. i really do think he is having a serious irritation due to the bedding though. i would consider taking him to a vet as well.