Google the Pelvic Instability Association to see if this is what you have. If you do, I recommend seeking a good osteo, doing some yoga (or continuing, if you already do it) and find a good acupuncturist for just in case it gets really bad.
Anyway, from here on in, really assess just how urgently something needs to be done and if it can realistically be left till you have assistance.
I don't believe there's anything negative that will happen to the baby, though there is some talk about stretching up promoting cord-wrapping...one of the 'old wives' tales' that I think I read last time *may* have some merit...grain of salt!
Trust me, problems to yourself are not something you want to invite when pregnant...or you'll spend more time thinking about the problems and treating them than you will in feeling pregnant and being able to enjoy it. I have SPD and DRAM (tummy muscles separated) and haven't been able to ride my horse since about 21 weeks...probably a good things, but I'm so sad that I can't be one of those preggy chicks who can still pull a dressage manoeuvre into my 36th week
Thankfully, time has a way of ticking on and I've got 3 and a bit weeks to go, give or take a bit and I reckon I'll be back in the saddle (but not lifting it!) by mid November, if not sooner
There are threads on SPD in the 'Medical Conditions During Pregnancy' forum. I hope you don't have it, and if you do, the website I gave you has some great management tools.