Do your feet sit on the floor at all times? Do you have support following the curve of your spine or are you just sitting on the pillow? The latter, especially with it on a hard surface, will contribute toward tailbone pain and discomfort. Tightness in the hamstring can ride up the buttock deep inside where the piriformis can feel tight. You can get a syndrome over time which consists of symptoms like dull buttock ache, sciatic pain running down the back of thigh, calf, and foot, pain walking upstairs/on incline, increased pain after prolonged sitting, and reduced range of motion for the hip joint. In any event, stretching, trigger point release therapy, self-massage, and resistance training through light weightlifting will provide stellar results. The single best exercise to free up the lower back and top of the legs is the bulgarian split squat. This is what I get my kids to do after sitting on screen for more than 40 minutes. Try it with a dumbbell or something that can double as one for now