So confused! Any input helps so much.

So I was 7 days late on my period. I took several tests over those few days and all negative. Finally started bleeding pretty heavy for one day, a little bit the next day. Enough to fill a tampon. But it only lasted two days! And I always have a normal 28 day cycle and it lasts 4 days. Any clue what could be going on? Can't go to my doctor because I am out of town right now so any ideas really help. Thanks in advance ...