Brave man, is this on your Jakstadt? I have had pain there from hunching my back or raising my left shoulder.
If that sounds like you, here is what worked for me:
Stand with feet between shoulder and hip width apart.
Hold the bass straight up and down an arms length in front of you. Adjust the endpin so that the heel of your hand can rest on the heel of the bass (you would be playing C and D on the G string in this area) often the bass is too high or too low.
Turn the bass very slightly and pull it back to your body. Resist the temptation to lean forward over the bass. I usually shoot for the corner of the bass to rest between where my leg and my torso come together. This sets you up for the next step.
As you ascend into thumb position, rotate forwards from where your legs come into your torso, moving your whole torso to give you extra length in your arms. Try to have your body stand tall over your bass, but let it be tall, don't try to overextend yourself.
Stop looking at your hands, let your head balance on your neck. Remember that thing weighs as much or more than a bowling ball and can add to neck strain, especially when you are staring at your hands.
Keep monitoring your body for problem areas. I was so bad at this I had to just pick up my bass and not react to it, I would just tense up as soon as I brought the bass towards me.
Pay attention to your alignment, watch out for where you start to rotate your torso or hold yourself too tight.
Also, maybe you are working too hard with the bow, this will add to your tension. Is your bow close enough to the bridge?
Good luck!