It is called tendonitis , or inflammation of the tendons.
There are some steps to help prevent the occurrance of that.:
Adjust the shoulder-strap so that the bass is in the same height when you sit as when you stand.
The bass should be positioned in a 40-45 degree angle i.e the headstock (the nut) should be in the same height as your head.
Shoot the neck-part of your bass a tad bit foreward , and push it somewhat towards your right (dependig wether or not you're a right or left-handed player).
The bass should now be in a position that you can hold quite effortlessly.
Make SURE you don't bend your right-hand wrist , instead push you elbow a little bit forewards , so that your wrist is no longer bent. Awkward at first , but that will quickly pass.
There are no muscles in your hand and fingers that you would use while playing bass. There ARE some muscles there , but they are only for support of the tendons and skeleton.
The muscle-group you use while playing is located just beneath your elbow-joint , at the front and back of your forearm. These muscles pull and release the tendons that are attached to your fingers.
The most common tendonitis problem is the one on the wrists , they occur because of playing with bent wrists. When you bend your wrist you tighten the joint , the blood-circulation that passes through your hand decreases and the oxygen-level lessens in the muscle-group that supports your tendons in your wrist and hand.
This causes a high level of stress to your tendons on the front and back of your palm and wrist. Waste-products such as dead cells and tissue aren't caught up in the circulation , and bacteria starts gathering around the sore points of the tendons. That's tendonitis.
And warm up! I usually start the warm-up by doing push-ups (on the knuckles)and sit-ups etc , this increases the blood-circulation.
You don't have to do many of each , just do it for like ten minutes or so before you pick up your bass and start doing other warm-up exercises.
Well , hope that helped , good luck.
Tyr