I'm wondering if playing upright has caused what I'm experiencing. I've been playing upright a LOT lately, and I've been having pain horizontally across my abdomen, and into my back left side. The skin feels like it 'burns' when touched. The line that the pain follows starts approximately 1 inch above the navel, and goes a little bit to the right, and from left to back. It's a 'close to the surface' pain. Anyone experience this kind of thing from shedding/playing?
My partner (who is not a bass player, but she has all kinds of experience with massage therapy, cranial sacral therapy, and a bunch of other stuff I don't understand) says you need not to lock your knees. Relax your knees, tilt your pelvis forward just slightly so you're rocking back and forth so your knees and hips are moving like they should, and relax your grip just a little. Somatically you're hurting your body because you're not relaxed enough. You could be wearing the wrong shoes. Try playing barefoot. Take that for what it's worth; I don't understand and can't explain it, and I tend to be a skeptic with things, but she has some amazing abilities that I can't deny.
It may or may not be related to playing more, and it sounds like seeing a doctor would be a smart move.
I don’t know how much you’re playing. I played 2 hours today in 35 degrees and the only discomfort I got was cold hands and feet. If you have a teacher you should discuss this with them.