In my last pregnancy, just 2-3 days before I got my bfp, I found I had flu like symptoms as well.. thought i felt feverish , there ws no fever.. I had headache, cold and simply could not get myself out of bed..
BUT, not for 3 weeks.,. that sounds like a long time.. maybe you should get it checked..
good luck!!
Honestly, it sounds like an actual cold. I've never heard of some of these being related to pregnancy. It also sounds like it started right after your last encounter, and it would take at least a week or two before the egg implanted and started to produce hormones that would cause symptoms. If they are related to pregnancy, you can test with accurate results at this point.
i would say test. you would have enough hormone to indicate. insight of flu symptoms. i would say it is 50/50 percent.