Positive but no BFPs??

Becka • 24 years old / Growing baby #1 due May 2018

So one day before AF is due. Had a faint positive three days ago and they have been getting slightly darker or if not, just showing up quickly but I still don't have my BFP. I'm 3 weeks and 6 days with my first ever pregnancy, have badly bleeding gums and mild cramps. Has anyone got any experience that could put my mind to rest?