What could cause a bump on the roof of my mouth?

Lump in the mouth: I hate to say this, but this is something you really should see your dentist for ... Such a bump could be normal boney structure that you were previously in aware. Or is could be an abscess - though these are usually associated with pain. It could even be cancerous. Unfortunately, your description and lack of a clinical picture ties my hands. See your dentist.

Not sure?: Anything from trauma to an abscess from an upper tooth can cause this. Need to check with your dentist.

Torus palatinius: If it is a bony bump it is probably something called a torus and a harmless. Your doctor or dentist can tell you for sure by looking.