The thing to remember is that although you are normally regular, you can have an off cycle. And it is completely normal. Holidays include changes in schedule and stress often. Both of these are known to mess up our cycle. Home pregnancy tests are very sensitive and usually produce a reliable result. Timing matters though. Do you get a false negative sometimes? Yes, it can happen. But usually timing of tests. Or you have not yet produced enough hormone to be picked up by them. But these tests do so pretty quickly usually. Normally, you can get an accurate yes or no when you test 2.5 weeks after the sexual encounter. Was your encounter unprotected sex, by the way? I would do this. I would wait a full week from when you last tested and then take another test if your period hasn't shown. To me, some of the things you mention sound like your period may be about to show up. Hope so. But if it doesn't, wait that full week and retest. Still negative, then don't worry. You aren't pregnant. Doctors want us to wait for at least 3 abnormal cycles before seeing them because most women have situations like this from time to time.
I DID develop a thyroid issue (working slowly) for about a year in my 20's and had very long cycles (like a period every three months). I went to the doctor after a period of time and was investigating why, did the thyroid test and found about that. But while waiting for an endcronologist appointment, it normalized and was regular from then on.