You need to see a doctor. If it is tendinitis, they can fix that up pretty fast. If it is arthritis, that could be a long term problem. I have had issues with both, but don't draw any conclusions from my stories.
About 12 years ago I developed chronic pain in my right hand. After a couple of months, I went to the doctor and he told me that I had tendinitis. He gave me some Celebrex and sent me on my way. I wasn't on that long enough to help. I discovered that I'm allergic to it, so I went back to the doctor. He said that I needed physical therapy. I told him I didn't want physical therapy, and couldn't we just splint it for a few weeks. No, he said I needed physical therapy, so off I went. For two months they hit me twice a week with steroid cremes, massage, and ultrasound, and every week I asked them to splint my hand, and every week they refused. Needless to say, physical therapy didn't help, and after a solid two months of it, they finally decided that they needed to splint my hand. I wore a splint for two weeks, which forced me to rest my hand, and at the end of the two weeks, the tendinitis was gone and it has never been back. The joke came later when I found out that my insurance didn't cover physical therapy and I was personally on the hook for $1200. How many basses is that, exactly?
Moral of the story: Rest that hand. Cut all nonessential playing. Splint it before you spend 8 weeks in physical therapy.
That's for tendinitis. Arthritis is messier. Overuse, injury or misuse of a joint can cause that, but so can systemic disease. Rest will probably fix the first problem if you're young, but if it is systemic disease, then you'll need medications to deal with the problem, and if you're young and have some variation of serious arthritis, that's bad. You'll need a rheumatologist for that. Those kinds of disorders will often start in one joint and then spread.
Given what you do, I think you've got an overuse problem and with some serious rest, you'll be okay, but you should see a real doctor since I only play one in the bedroom.
