Spotting instead of period. What could it mean?

Mackenzie

So my period was supposed to start Dec 14. Dec 15 I had spotting. When I say spotting I mean one single drop of brown blood. Nothing after that. I took a a few pregnancy tests and all came out very negative. I haven’t been feeling that great but there are a lot of factors to that. I’ve also had a very regular period for many years. I’ve never had this happen before so I’m just confused and wondering what I can do to understand my body better. I do work night shifts but I’ve been doing them for more than half a year and still always had a regular period. If you have any insight it would be greatly appreciated!