I had it starting around 9 weeks. Actually earlier, but 9 weeks was when I saw my doc and figured out what was going on. I was shocked too, there just isn't that much growing going on yet! But your uterus has already gotten quite a bit larger than normal, and there isn't that much room down there, so it is just sitting wrong.
My doctor recommended physical therapy, chiro, and I have also heard massage is good. (has to be chiros and massage specifically for pregnant women to be safe). PT is usually covered by insurance, chiro is sometimes, massage is usually not. I recommend getting some sort of help, since mine still hurts 2 1/2 yrs later. I should have taken the PT when I had insurance that would cover it even if it was only partly. Now, I have to meet my deductible first and since I don't have lots of medical bills, I won't do that for a long time. There are some stretches you can do too- Pelvic Tilts and some yoga poses (the one where you are on hands and knees and arch your back upward then straighten). If you see a therapist, they can show you how to do these. Definitely ask the doc for things you do at home too. Good luck, I know how miserable it can be!