The most common reason for back pain during the p.sit is too much of an arch in the lower back and not enough engagement of the deep lower abs.
Modify your moves
The p.sit should be a hinge from the hips, meaning a bend in the hips and not in the back. Try keeping your lower back in a neutral, lengthened position and tighten up the lower abdominal muscles, drawing your belly button in as if you were zipping up a tight pair of pants. It helps to always think about keeping your front ribs and hips knit together.
Keep this spine position and core activation as you go into the p.sit. You can also try slowing down and taking the time to really feel your muscles create the movement— not momentum. Keep your p.sit shallow to start, and over time, you will be able to get deeper into the position. Keep in mind that you are engaging your back muscles here, but with these tips,your back muscles will get stronger, and you'll only feel the movement in your glutes!