I would first contact your doctor to make sure that everything is ok with the baby and there is no medical reason why your stomach is aching. If everything is fine then take a nice wark bath and relax. Have a glass of water and lie down on your left side with support under your bump and around your back.
You could also do this with a hot water bottle under your bump. Constant stomach ache is also a sign that labour is on the way so ensure that your hospital bag is packed.