Pregnancy causes a woman becomes more sensitive about the condition of her uterus. The air in stomach can cause light pain that might felt like the fetus is sliding inside her uterus. Some women feel her uterus throbbing when she gets the pain. According to Sears, this condition happens because of the increase of blood flow. There are a few things that could cause uterine muscles getting weak such as heavy work outs, hunger, and also stress. When the uterine muscles get weak, that’s when the pain appears.