My hamster has weird WOUND on his back

[Alvin]

Hi guys, I don't know what to do. Yesterday when I took my hamster out of his cage for night play time, I realised that he has quite big wound on his back with missing fur. He didn't have it the day before. I'm working from home and his cage is near to my table and I didn't notice any weird sounds. But it's true that he has hobby - to try fit himself into places which he cannot fit at all. His cage is safe but he still finds way to stuck himself somewhere.. But this wound is just only on his back it has oval shape. It's dry. I thought about some skin disease or allergic reaction, maybe he scratched himself or bite himself. But he didn't have it day before and his behaviour was and is normal. He is still very active,doesn't scratch,or bite he is drinks and eats normally. He has the same bedding. Only thing that might cause allergic reaction is that once I played with him on really old bed sheets. But that was just for like 10 minutes.
It was really hard to take the photo because he is really active but I tried my best. The wound is not so red in real life, is more like orange and not so vivid.
What do you think? Should I go immediately to vet or it's just scratch or from him being stuck somewhere? Any home remmedies? He is half year old, russian campbell. Please help I'm really scared.

ps: sorry for my english

[cypher]

I t definitely looks as though your ham has been rubbing against something, so check your cage carefully & remove anything that has holes too small for him to get through easily or anything that has sharp edges, what size wheel do you have? Sometimes they can get wounds like that from rubbing against the spindle in a wheel that's too small.
If the wound is clean it may heal ok but you will need to keep a close eye on it in case of any infection in which case you would need to see the vet.