I feel for you! Could be Pelvic Girdle Pain (or Symphysis Pubis Dysfunction) I had it during my first pregnancy, lasted from 20 weeks until delivery. Here is a brief article that describes it : http://www.nct.org.uk/pregnancy/pelvic-girdle-pain-pregnancy
I found that the SI-Loc belt (http://www.amazon.com/Maternity-Si-Loc-Support-Small-Medium/dp/B0019ZYXWQ) helped stabilize the pelvis. Unfortunately, it is a bit bulky for this summer weather.
The biggest thing that helped me then (and perhaps has prevented it from happening with this pregnancy) is keeping legs/knees together as much as possible. So that sort of eliminates many yoga poses (I did a lot of yoga before the pain came, it exacerbates it by furthering separating the joints). When you get out of your car, swing your legs out together as opposed to one at a time. When sitting on the floor, sit with your legs out long in front of you, or go from high-kneel to sitting "on" your feet (does that make sense? I can't think of another way to describe position.) Swimming is the best exercise and I have heard of people feeling great after seeing a chiropractor. I saw a physical therapist who gave me exercises which helped for short periods of time.
I really feel for you, I remember not being able to walk, or go up stairs without excruciating pain. I heard that it gets progressively more common the more children you have, however, my experience has been the opposite. The moment I found out I was pregnant, I was super cautious about how I moved my body.
Hope this helps, and I hope you are able to get relief!