...by Sunday evening. One of my kids did the same, although their test was antigen, not PCR.

I woke up Monday morning and was definitely sick -- fever of 100.5, achy, scratchy throat, etc. By Tuesday, the fever was gone, but I still had aches and a sore throat. Ditto for yesterday. My head has felt stuffy, but I don't actually have to blow my nose. I feel better today than I have since Sunday. Not totally back to normal, but pretty functional.

FWIW, I've heard that this new COVID (if that's what you have) is more of a throat thing than a nasal thing, so people are saying the nasal swabs aren't necessarily picking it up, leading to false negatives. So, one doctor we know here is doing throat swabs instead. If it doesn't bother you too much, you might try that for your next test just in case.