Spinal adjustment would be suggested. Acupuncture or chiropractor may help. Ever deep sports massages. Does sound like a nerve displacement problem.. Any burning might suggest slight muscle/tendon tear. Even neck and traps will need attention, in some cases.
Is the saddle equal or mounted well? If one side is higher than the other, or if having anything in your back pockets, can cause a unequal /unbalanced shift of bwt, affecting the spine. Just about every negative cause factor can be related to the spine & hip adjustments, or lack of. Having undergone a serious spinal operation, I have become aware of the important of the spine/lower back.
Could be related to your weight training. Any new exercises or training protocol , I wonder. Good luck.