Unexplained fever with dizziness and hot flashes?

For the past month I have been running a fever of around 100.9 101.8 being the highest. When I stand I get really dizzy and have hot flashes. I'm also extreamly fatigued. I'm not sure if this corralates but I also have a really bad pain in the bone that runs down the left side of the back of my neck. [I'm 20 so it's not menopause and I know for a fact i'm not pregnant.] I went to the doctor and he's done blood work and a chest xray and told me to 'just take it easy and call if anything gets worse.' Any ideas on what could be going on? Thanks.