I had a bump on the roof of my mouth for over a year before doing anything about it. Since there wasn't any pain and it wasn't getting any larger I didn't worry about it.
It turned out to be cancerous and I had to have the lump surgically removed. It turned out that under the skin, where I couldn't feel it, the lump was about the size of a quarter. It was a slow growing type of cancer, which was a good thing since I kept putting off having it looked at. The oral surgeon had to go all the way down to the bone on the left side of the roof of my mouth. He had a special appliance made up ahead of time and I had to use that as a false roof of my mouth until it healed enough. It still hasn't completely healed after eight months.
At this point I am going to a specialist that deals with head and neck cancer and I am having to have a head and neck mri every three months to make sure that it hasn't spread.
Coming from what I have been through in the last eight months, I would tell anyone that has a lump in their mouth that doesn't hurt to get it looked at by an oral surgeon, not by your normal dentist and don't wait over a year like I did. What they will probably do is a biopsy (they will cut a very small piece of the bump and send it to a lab to have it tested to find out exactly what it is).
I had gone to my regular dentist for check ups and when I had him look at it the first time he gave me a list of things, "it could be this, or it could be that" then at the very end he patted my arm and said "i don't want to scare you but it could be cancerous" but he said it in such a way that I didn't feel concerned at all.
Please learn from my mistake and have it looked at soon. You would be better off to go and have them tell you it's nothing than to not go for a while and then find out it is cancer like I did.