I'm sorry you're suffering so. And really tough during pregnancy as there's nothign you can take. I also have sacroiliitis and have had it for years. Ironically mine was the best ever during my pregnancies...guess the hormones were for the good. It's pretty bad now with daily pain just from sitting (I have a desk job). Get flare ups if I try to exercise, sit too long, lie on my back, lift something heavy (which is tough to avoid as I have an almost 2 year old), etc.
I do take NSAIDs or prescribed celebrex when needed, but of course you would need to wait until after the pregnancy for that. You can try ice every 20 minutes to reduce inflammation.
They can do xrays and mri to rule out AS, which it can develop into. Hopefully it's not that, but if it is, there are meds to take.