The Last Time I Had My Period Was November 16-19.

The last time I had my period was November 16-19. On December 16th I had a light pink discharge for about an hour. Then January 19th I had super light spotting that was rust coloured with some minor cramping then it went away, on the 20th afternoon it came back and was super dark, finally, on the 21st it was almost black. On all three of these days it was so light I would almost consider it spotting, as I only needed to use a liner. Normally I have a pretty heavy flow and am quite regular. I have taken several home pregnancy test and all have come back negative. What could be the reason behind my missing periods?