Mine has started a bit, too. Last time it didn't until nearly 40 weeks and postpartum it was bad, but 20?! I started sleeping with the pillow between the legs this week.
I think mine was set off by overstretching in yoga--we were doing legs up the wall, and went into cobbler (knees open, feet together) and my instructor (not prenatal, a regular class) encouraged me to open my legs wide (I am very flexible and sometimes she puts me in different postures to achieve a deeper stretch). Anyway, I got a great inner groin stretch, nothing that felt bad at the time, but the pubic bone pain began that night, so I think I need to be careful about pulling apart in that area.
I know deep squatting is recommended (especially in Bradley method) but I think that is something to be careful about, too, if you have pelvic pain. I'm so bendy I can just drop down too deep and pull those bones right apart.
We also have to be careful about positions during, ahem, intimacy
Some can cause it to hurt more afterward due to overstretching. I am naturally flexible to begin with, and I think I make, like, twice the relaxin of a normal pregnant woman. It was a bonus when birthing my 9 lb. baby, though!