Your body produces hormones to help your hips expand to accommodate the baby passing through. This is why some women's feet also flatten out and get wider during pregnancy, the joints are all relaxed.
I have no idea what SPD is, but what you are experiencing is very normal. Painful, yes, but totally normal just the same.
A belly band is a GREAT idea. I had the kind that was more like a belt to hold my hips together, I would cinch it up as tight as it would go so that it my hips wouldn't twist and press on my sciatic nerve. I had the extreme pain from the time I was 9 weeks pregnant and I did physical therapy (made it worse), I did exercises (I don't think it helped at all), the only thing that eventually helped was delivery.