www.pelvicpartnership.org.uk
Exactly what Palcum said. In me it's linked to hypermobility but really it's pgp as the other elements have settled. Ribs are still a bit bad though. It's mainly my si joint.
Periods are really bad times - I didn't know relaxin is released then too and I think the other hormones can increase inflammation etc.
Stopping bf hasn't made a jot of difference - it's keeping strong that matters, so a really good physio is essential. I found a sorts physio good who I see through work insurance.
The tummy issue won't help as you need to help your core support your pelvis, so Pilates and specialist core stuff will help there.
The most important exercise aside core leg raises I've been told by my physio is side planks and I believe they help the DR too.
Do find a good physio asap.
Katy Bowman has written a lot about both issues and published a book plus physio videos if you want to look at self help. My physio also adjusts my pelvis which drops slightly one one side and stiffens up. This also impacts upper back / shoulders and he's given a fab relaxation exercise where you lie on a rolled up towel in your upper back to around your bar strap area. Katy Bowman also recommends that one.
(I've basically got the t shirt! I was worse though as became very weak post baby due to other systemic issues)