Sounds like SPD to me! Get thee to a physio (you don't need a doctor's referral). I have been seeing a physio from about 18 weeks and I think it's made a huge difference, not only in that you feel better when they've worked on you , but they also tell you what to avoid to stop aggravating it, and teach you some stretches and exercises to strengthen and help. They can also give you a support belt thingy which helps.

Get onto it now because you don't want to end up on crutches before you have your baby!