I have had a low grade fever (99-100.5 degrees F) for the past 5 days along with sinus pressure. Whats wrong?

I'm a 17 year-old male, and hardly ever get sick with fevers or anything. The first day i decided to take my temp my body was achy and i had been EXTREMELY tired for 2 days. I never feel to bad, despite the sinus pressure, and always have an appetite. I have cranked out some green phlegm and am wondering if maybe this is a sinus infection and not mono? please help me out asap