It has to do with your body preparing for baby. All your muscles and ligaments stretch and soften to make room for your uterus. This is why they don't recommend women lift heavy objects in pregnancy. The baby is protected but all your muscles have slackened. Take it easy for a few days and try to relax yourself. If in a week or so you find that the pain is still bad then check with your dr or ob.
Hope you feel better soon