Aih mama, at 36+6 weeks with frequent tightening and back pain like this, you really should call your hospital or go in to get checked out, better safe than sorry, especially if you can't move when the pain comes. Don't wait until 37 weeks lah, a few of us preggy friends had similar symptoms and our doctors said it's important to rule out anything serious. Your body is telling you something, so listen to it and get yourself seen today or tomorrow, okay? You'll feel so much better once the doctor gives you the all-clear. Jangan tahan-tahan alone at home, sis!