SI joint pain after low speed impact

Hi all,
After giving birth in 2012 I started experiencing severe pain in my lower left back and hip area. The best way to describe would be like a hot poker twisting in my hip. I was misdiagnosed and mistreated by the NHS who said I was just stiff. I went to a private osteo and it was diagnosed as SI joint pain. 6 sessions later I was sorted. I went for 18 months with no pain.
In March this year I was at the front of a line of stationary traffic. I was leaning forward and looking right to see a gap in the traffic. The guy behind pressed his accelerator instead of brake and went into the back of me. About as low speed as you can get but enough the bend his reg plate and split the foam in my son's baby seat. The pain came back in my hip area that same day and took 6 months to settle. His insurance company are disputing the severity of the injury despite me having to have 8 weeks of physio, saying I couldn't have that level of injury from such a low speed impact. I know I'm telling the truth. Is anyone able to reassure me that this injury does happen in low speed impacts?
Thanks in advance.