Why Can't I Throw Up?

Okay so last night I felt sick and felt like throwing up but I didn't and it took me forever to get to sleep because I still felt sick. So all of today I have felt feverish. It's now night time and I just had diarrhea and now I feel like throwing up but I won't! And I don't want to force myself to throw up because it wouldn't feel right. Why can't i throw up??