If a person has a bump inside their mouth is that a form of a disease?

Hard to tell. A bump can be anything caused by irritation, infection, inflammation. Have a doc take a look.
Depends on the bump. There are lots of things that can cause a "bump" on the inside of your mouth. These can range from a plugged salivary or mucous gland to an abscess to oral cancer. You should see your doctor to figure out what this might be and what should be done for it. You should also describe this in more detail, how long has it been there, is it tender, hot, etc., and why is it bothering you?