severe pain in back on pelvis / lower spine upon lying flat

[firstchoice]

I wonder if anyone else has this and could give me some pointers?

I had two difficult pregnancies with really severe symphysis pubis disorder which left my hips wobbly.
This was some time ago (6 years) and I am told my SPD is resolved now but I cannot lie flat on my back as the pain across the back of my pelvis / lower spine is really severe.
I feel like - prior to any problems - I was like an open clam shell, pelvis wise, now I feel like it is partially closed and will never open flat on its hinge again, if that makes sense.

does anyone know what this could be / how to help?