Bass isn't a guitar, and if you try to play it like one, this is what can happen. The way you're playing is the culprit. That's why I use the Carol Kaye method of playing with a pick, which is detailed on her website under "Player's Points" or something like that.
www.carolkaye.com . It doesn't look as cool as that strumming thing that young bassists think looks so awesome these days, but you won't have to ice down your hand after every show or have surgery in a year.