Posterior pelvic pain

Anyone else having poster pelvic pain? I have pain and stiffness on my right side in my buttock and low back and back of my pelvis area. No pain or discomfort in the front. I'm sure this is just part of it but just wondering if anyone else experiences this type of pain and if anything you have tried helped? Stretching and massage feels great but so far hasn't improved the pain. I wonder if it has to do with baby's position? I am 35 weeks 4 days and have an appt with my OB tomorrow, and also an ultrasound to check growth so I'm sure they can tell me position too.