Sounds like you need to adjust the pegs. They can be a bit tricky to get them "just right." It's not a one size fits all deal.
I've had a bad back for 30+ years. PT's & Dr.'s tell me to keep my knees slightly above my hips when sitting, driving or riding. I suggest you start there. Remember angle of your feet / toes is also very important. I ride with my toes in a relaxed & a natural position, not pointing up too much or forward / down.
Also remember to change position back & forth between the two sets of pegs. At a safe speed, you can also stand up on the pegs for a few seconds, like on a dirt bike. You get some air in your jeans & cools your butt. Give the above a try, might work.