I experienced the same through my first pregnancy, and started feeling it with my second this week (9 weeks). I find it helpful to lay down and cross my effected leg over my other leg (that leg is straightened) and pull the straight leg towards me. Similar to creating a 4 shape with your legs. That stretches those muscles in lower back and buttocks area really nicely. Hold for about 30 sec and repeat. I do this every morning and night. Prenatal massages are very helpful too. Tell therapist what you're experiencing and they can dig in and help release the muscle tension.
Ive experienced sciatic pain off and on the last two years from a sports injury. My chiropractor suggested doing squats (even while pregnant) to strengthen and stretch the muscles in that area.