It feels like there's a small lump below my belly button. it isn't increasing in size and it doesn't hurt so i'm wondering what it could be? pressing on it or rubbing it makes the skin above it slightly tender?

There are many possibilities including a cyst, a small skin abscess, etc. The only way to diagnose it for sure is to examine it so you should see your doctor to evaluate you and get whatever test is needed to diagnose it. A dermatologist may be helpful in the evaluation also. So, get it checked by your doctor so that it can be diagnosed and treated. Good luck!