I ate some food last week in my school's dining hall and It gave me diarrhea. I thought that it was just because of the mixtures of things that I ate. I had diarrhea for 2 days or so. But,my stomach has been upset since then, and that's been for about a week now. This morning I went to the bathroom and my stool had dark spots in it,they almost look black. The stool was soft. I feel nauseous,a little dizzy, my stomach hurts, and I have a headache. Can someone help me,please?

I am a 23 year old female. 5'6" & a 1/2. I'm about 155 pounds.
Medical History
I have pollen allergies. I've had inconsistent periods since I began them. I am sexually active,but monogamous. My family has a history of heart conditions. On the other side of my family,there's a history of breast cancer. My maternal grandmother recently passed from it. At age 17 I was thought to have had a heartattack,due to stress. I tend to get stressed a lot. I have a healthy appetite. I don't use drugs,smoke,etc.