It seems ok, but what's the context? If the asker was talking to someone who just fell over or who was just in a fight, it would be fine.

If you really just bumped your head, I think it sounds more natural to say: "Yeah, I just bumped my head. It's no big deal." If you want to use, "Just a bump on my head", you can use it in this context: A: What's that on your head? B: Oh, just a bump on my head. It's still sounds a bit weird, but what is the meaning you are trying to convey with using the phrase?