I've been having weird symptoms, what could this be? PLEASE ANSWER.?

This morning I woke up to my alarm clock, feeling okay, but extremely tired. I just couldn't make myself get up. So I went to go find something to wear to school, and realized, standing, and sometimes just sitting up, my head feels very congested and dizzy. I had pretty bad diarrhea earlier, though I'm usually very constipated. When I stand it becomes so hard to think, I get really dizzy and nauseated, though I haven't thrown up. I'm not running a high fever. My body temperature is around 99.1. I'm shaking a lot, and I took advil, hoping my head would feel better, but it didn't do much. And on top of this, I feel EXTREMELY worn out. I've slept almost all day, and I'm still tired. What could this be? People have mentioned anxiety...?