What Causes Missed Periods Along With Headaches And Loose Bowels?

I have missed 3 periods and I feel sick at random times of the day. It feels uncomfortable to apply pressure just below my rib cage making me feel more sick. I also have episodes of very loose bowel movements. And have had headaches more recently making me feel light headed. I have taken 2 pregnancy tests over the stretch of 3 months both negative. What could be wrong with me?