Hi it could be that you have torn a tendon or there is another problem with the thumb where the joint is affected. If I were you I would see a doctor. Wearing the brace is a good idea to provide support as well as remind you not to use
Good luck
Hi there!
Well, without a clinical evaluation it would be difficult to determine the cause of the lump. Possibilities that may need to be considered include ganglion cyst, lipoma, neurofibroma, growths from the bone/ surrounding connective tissue etc; while the pain could or could not be related and could occur secondary to injuries, infections, inflammations, degenerations, neuro-muscular issue etc. I would suggest getting this evaluated by a primary care physician or an orthopedician for an accurate diagnosis and appropriate management.
Hope this is helpful.
Take care!