It could be the ligaments that hold your uterus in place. They get stretched as the baby grows, and sometimes they hurt. If it's the ligaments, it's normal. Talk to you OB at your next appt. to make sure.
Probably just stretching like the pp mentioned! I swear I knew when my kids had growth spurts in the womb. I would get out of bed in the morning and sharp pains would shoot across my belly as everything stretched. I always felt heavier too! Lol!