I've had back pain (posterior pelvic pain, situated over my sacrum for the most part) on and off with this pregnancy since 5 weeks! For whatever reason my body/joints/muscles/ligaments are responding differently this time 'round. I'd call your doctor to make sure, but it can be 'normal'. Sounds like it might just be the norm for you too given your SPD, but still, I'd play it safe and talk to your doc.