What Causes Persistent Headaches, Fever And Weakness?

I am a healthy 48 yo female. I have had headaches, fever and weakness for the past 6 weeks. I do not have insurance, I went to a care now facility at 4 weeks, they could not find anything wrong w/o doing tests. They prescribed me amoxicillin, I started feeling better and finished the prescription, but my symptoms never completely went away and now I have them everyday again. I have taken acetaminophen everyday 2 or 3 times a day, I cannot take ibuprofen or naproxen sodium due to a sensitive stomach. I have taken Nyquil and a sleep aid at times. I have spent a lot of time in bed and I feel that has contributed to feeling weaker and to a backache. I have just started doing yoga stretches for a sore back and it has helped.