Feeling sick

For the last month I've been feeling sick at my stomach constantly. It's like a motion sickness feeling but it's constant. Light headed and dizzy a lot. I feel like Im gaining weight and that my stomach is getting bigger but when I weigh myself it doesn't change really. And I feel like something hits my stomach sometimes. I have sex quiet often, always use condoms, and didn't miss my period last month. I should start my period tomorrow. What could this sickness be?