Need help for the cause of my Goldfish's death.?

I picked up 2 gold fish 3 days ago. (different type of goldfish, one is larger than the other, and one has a little cloud looking bump over its head) I found the little one floating dead on the surface. I dont know why, they seemed perfectly happy in their new 10 gallon filter tank, it was really shocking because nothing seemed wrong. The other fish is doing great, swimming all around, picking on rocks, so i dont think it had to do with the water. I want to know possible reasons why the little one died. I tried to pull a little autopsy, to see any obvious cause of death like a jammed pebble or something, but i did notice that her stomach was bleeding.