Low grade fever for 3 weeks that isn't an infection

18/M

I've been suffering from a low grade fever of around 37.3C to 37.5C for the past few weeks.  I've been to a GP and at first he told me it was anxiety and gave me some meds. I didn't get much better so I went to the GP again, who gave me some different meds, and after taking those, everything but the fever was pretty much fixed, aside from some occasional aching and headaches. I went to the GP again today, and he tweaked the meds again, but still doesn't really know why I still have a fever.  I had a CBC blood test, and the results came out fine, and the GP says the blood test shows I don't have inflammation.

I've taken even more meds and the fever still hasn't broken yet, it hasn't gotten any worse or any better.  I also occasionally feel light headed, and my muscles often twitch, but that could be because of the medicine that's supposed to make you fatigued.

I'm getting really worried becuase I still don't know where the fever is coming from, all I know is it isn't from an infection, which is freaking me out even more because i'd rather get an infection since they're easy to treat.  I'm thinking that i've gotten cancer and I don't know what to do.  I don't know if I should schedule another appointment with the GP or like get a full body MRI scan to make sure I don't have any tumors.