Pain is always an indication that you are overtaxing your body in some manner. You must use efficient form and develop the muscles required to play the bass. It is not difficult and it doesn't take too long to develop enough strength to play the bass... but it can take a long time to learn proper and efficient form. Many players try to "muscle" the bass. Better to focus on efficient angles-of-approach and form. See previous threads regarding these topics. Also, view a lot of players (start with my vids on YouTube - channel is "PNeher") and try to emulate the ones that you think are playing with the most ease. Performers like Christian McBride, Francois Rabbath, John Clayton, and many others play with ease and NO PAIN! I've been playing bass for 46 years and if you develop and use the proper-for-you and efficient-for-you technique, you will play at least as long as me! Best of luck!