See a doc that specializes in wrists. Bring your bass. Demonstrate. He'll figure it out.
The longer you wait, the worse it gets, the longer it takes to cure. Stuff goes through narrow passages in your wrist. You irritate them because of your technique. They're rubbing against something. They get irritated, the swell, and now the space that was too tight before is even tighter. Its a vicious cycle, and feeds on itself. Technical term is impingement.
After you play, ice down the wrist. Take motrin or other nsaid to help the swelling.
Also, should cut down on your playing to give it a chance to heal. May have to cut out completely for a while depending on how bad it is. You need to gradually build up to playing a lot.
Concentrate on playing with a light touch also. Get lessons to make sure you are doing it right. Just a couple to make sure your hand positions are good.
And if it only happens when you sit down, STOP SITTING DOWN. Play standing up. Sitting is putting the bass in a different orientation that's causing extra strain. Stop irritating it or it'll never get better.
Randy