Tracking period questions

I used to have a regular by the books period before ever getting on the pill, I went off it once and everything was regular, went back on it again and regular. Now I've been off the pill since November and my period has been all over the place. I think I'm 12 days late but I can't remember when glow says it was supposed to start. Is there anyway to go back and check? Plus, the last time my period was missed and then the next one ended up being late glow suggested when to take a pregnancy test. No suggestions now. I honestly don't think I am pregnant but I am late (not entirely sure how many days late). Someone help! What should I do? There has to be a way to keep track of how late I am so I can accurately test