So confused

This morning was my predicted day to start my period. It’s cycle day 37 and my last three cycles have been exactly 36 days. When I went to the bathroom there was a very, very small clot but nothing else. I put a tampon in about two hours ago and just went to change it and it was completely dry. I’m not sure what this means!

I took an HCG test and a digital pregnancy test this morning just for the heck of it and they both came up negative so I’m not sure what is happening.