I'm having pelvic back and low abdomen pain for about 3 days now. I have a 99.8 fever. Also pain around the belly button. Stomach is hard and swollen.

See a doctor ASAP! There are several possibilities for symptoms like these, many of them potentially serious. If you are sexually active, pelvic inflammatory disease (PID, fallopian tube infection) due to chlamydia or gonorrhea is high on the list. Others include appendicitis, severe urinary tract infection, diverticulitis, endometriosis, infected ovarian cyst, and others. Don't mess around with this: get care now!