Af due today! Help..

Jasmine
Looking for advice. I haven't been on the pill for around two years now so me and my partner were using the pull out method to prevent pregnancy. However on the 6th of January we decided to "see what happens" because we both said we wouldn't be disappointed.. We had sex only the once on the 6th of January then I found <a href="https://play.google.com/store/apps/details?id=com.glow.android">GLOW app</a> and have become obsessed into thinking I'm pregnant, glow predicted that I would be due on today as I average at a 28 day cycle and turns out I was ovulating the first time we officially "tried".. Anyway today I haven't come on my period however I've had really bad cramping over the last week and I feel different.. I've been wearing a pad just incase I come on and I've had 2-3 dark spots over the last 2-3 days which look like dried blood.. What do you guys think.. Could I be pregnant after trying just once?