What Causes Fever,chills And Abdominal Cramps?

I am really worried. I currently have a fever of 100.8, and have had the chills all day. I have been wearing layers of socks, pants, etc. but only after napping was I able to get warm and it was then that I removed the blankets. Now, I have a fever.
Also, I am late on my period. I have not been sexually active for over 14 months, so there is no need to consider pregnancy. I do, however, have discomfort and cramping similar to when I AM menstruating. Thank you so much for your time.