What Are The Symptoms Of Flu?

Hi. I have thyroid cancer and am taking synthroid 125. I ve been on it for about 1 year 1/2. My lab results have always been good. Just recently, the past couple days I felt sick, but not flu like symptom sick. I felt really tired, lower back pain, headache only when I bend over, not a regular headache. No appetite. So, I knew it was something other than the flu. That s why I starting thinking my meds could be the problem. I m feeling better 5 days later. Not 100%. I still have some lower back pain but only when I m laying down. My question is, do you think this is at all related to synthroid or maybe something else? I would say it was some virus but the back pain is weird.