At what stage does back pain in pregnancy get considered SPD?

I have horrible back pain.
If I lay on my back, it seizes and I am not able to turn or get up unless DP helps me.
I can't lift one of my legs because my lower back/hips hurts if I do. So I have to put underwear/socks/shoes/pants on while sitting down. I can't bend down to pick things up off the floor either.
I can't walk fast at all, if I do my lower back hurts so much I can't handle it or my pelvis hurts too. I can not make sudden movements, so standing up from sitting takes double the time it used to because if I make a sudden movement my back seizes up.

I spoke to my GP about this (I am under DP care for pregnancy ) and he just laughed at me and told me it was normal.

But I don't think it's normal that I am not able to get up myself when I lay on my back. I was on my back for a hour the other day becaue DP was outside and I had no one to get me up. I was in so much pain just moving my legs a small amount to try and get up.