Go to a chiropractor!!
I have gone to a chiropractor religiously since I was 12 years old (I have severe back issues), but I swear by her now that I'm expecting. It is most likely strain from the muscles and tendons that are stretching to accomodate your growing uterus (yes, this is a direct quote from my chiro b/c I was experiencing the same thing!).
Also, a suggestion from my chiro, was to massage your round ligaments for about 60 seconds every morning before getting out of bed. You round ligaments are right at the top of those v muscles on your stomach (the obliques)... if you point to your belly button and trace down towards your hip bone, you'll feel the tightness there. I do this every morning and my chiro does a deeper massage weekly... and although I still do get minute tailbone pain, mostly only after I've been sitting for too long, I am sure it's not as bad as it would be if I wasn't doing these things!