Hello razi, sorry to hear about your current situation. Sacroiliitis is a common cause of low back and buttocks pain and unfortunately it does occurr more frequently in pregnancy. As your pelvis becomes larger throughout pregancny , in anticipation of vaginal delivery, the sacroiliac joints or the pubic symphysis can become inflammed and cause pain. Now having said that, having joint inflammation is no reason to not have a natural vaginal delivery. Your bes options for pain relief at the current time will be regualrly using tyenol throughout the day and to perform stretches. The best strecthes would be lumbar spine conditioning stretches. You can easily find these on-line or you can ask your primary physician for a physical therpay referral. I hope you feel better soon and good luck with the rest of your pregnancy!