What Is That Growing And Tender Bump On The Inside Edge Of My Thumb?

Question: Hello, A few months back I started having some pain that I thought might be tendonitis. Pain was stemming from my left thumb joint and radiating up my arm just a few (maybe 5) inches past my wrist. Mostly hurt when I tried to stretch my thumb inwards towards my pinky finger across my palm. So I just did stretches and anti-inflamatories to help relieve the pain. Well it didn't really do anything. I also iced it. No relief. But I am a person of high pain tolerance and just put up with it. I just figured it was from use of my cell phone and typing a lot at work. Well then it started to feel a little better, but at the same time a bump started to emerge on the inside edge (thumb side) of my wrist bone. This bump feels as hard as bone. So I am guessing it is a bone growth. It feels like a bruised bone too. Very tender. And seems to be getting a little bigger and remains very tender and weakened. The growth is about 1-2 cm in diameter and probably 5mm high. And my right wrist has nothing of the sort. Just curious if this might be something serious that you are familiar with?