What is wrong with me?

I had stopped taking birth control pills about 6 months ago and just started my period 3 weeks ago and i only had it for 3 days which is weird for me because when I used to have periods in the past I would have it for 7 days but anyways I downloaded this app to track my period and now I'm 6 days late for my period. I don't know what's going on. I had pregnancy test done at the doctors and I took one home but all of them say I'm not pregnant. Is there anything wrong with me?