As your pregnancy progresses, your uterus is also expanding. You might feel some tightening or known as braxton hicks as early as 16 weeks. It comes and goes for a few seconds (you will also feel this in the 3rd trimester as your body is preparing for labour). It will go away when you change your position ie. from standing to lying down. Also when you're in the 2nd trimester which is now, you will feel some ligament round pain at the sides of your belly. These are all normal but not too painful until you cannot bear. If it's too painful, call your gynae or head to the A&E.