I'm just getting over a case of tendonitis in my right (plucking) forearm, between the thumb and elbow. I had 2 months of 1-2 gigs every weekend, and I believe the high volume of playing, and my preference to play with the bass up high with a bent right wrist, caused it.
I made some dietary changes recently which incidentally helped with the tendonitis, and sore joints in general. I was already eating healthy food, but I increased my vegetable portions, while reducing portions of animal stuff. I'm surprised by how much this simple change improved how I feel. I'm 53 years old, btw.
I lengthened my strap so the bass sits lower, which allows me to straighten out my right wrist somewhat. I also try to avoid resting my forearm on the bass, which naturally leads to a bent wrist. Right elbow out.
Finally, I allowed it to rest. I limited my practice sessions to 30 minutes, and took a day off every 2-3 days.
After doing the above for 4 weeks or so, the tendonitis is nearly gone, and I've grown accustomed to the new playing position. Fingers crossed that it doesn't come back.