It could be the baby is positioned. Long as your not bleeding and the pain doesn't increase when you walk you should be fine. Drink some cold water and lay on your left side.
Could be round ligament pain, that's what it was for me before. It's common since the ligaments around your uterus are stretching, but I was practically bent over and could barely walk for a while. Resting did the trick for me. As time went on, I didn't feel that intense pain anymore, but I would call your doctor just to be sure, possibly get looked at.. but yeah, as long as you're not bleeding it could just be a normal side effect, everyone is different. Keep an eye on where your pain is localized though.