Hi, last year I had a filling which the dentist told me was quite deep and nearing a root canal. Since then I noticed a lump very high up on my gum which caused pain when I press it. I went back to the dentist last December and they said that I’m holding onto the tooth until I need a root canal and to expect some inflammation and occasional pain. However, it has been 2 months since them and it is exactly the same as when I last visited. The lump is very hard and very high up on my gum. It doesn’t hurt them gum when I apply pressure but the tooth does hurt. The tooth also hurts when I push it. You can’t see the lump in my mouth as it is so high up but you can certainly feel it! I know I shouldn’t but everything I google about mouth cancer is usually about ulcers and patches rather than lumps but also abscesses are usually soft and very painful! I am currently waiting for an emergency dentist appointment through my dentist but I’m driving myself crazy! 
If you got through my rambling, thank you!