Dear friend,
This is nothing to worry about as long as it does not become infected or sore. It is normal for Hammies to develope skin discolorations as they age. He may have gotten an irritaion from his wheel or a piece of bedding. You should only use carefresh (paper base).
If it gets cold in the room where he is kept - he WILL hibernate so do not be surprised about that.
Here is a link to hamster care:
http://www.hamsterhideout.com/hamstercare.html
Please let me know if you have any more questions about your sweet little guy. I will be happy to help.
Warmest best wishes,
Terri