Confused

Ok so on august 19th-23rd I had my regular scheduled period and according to glow I should be in my fertile window right now but today I had a really strange feeling that I am pregnant mostly because I can't go an hour without having to pee and the last time that happened to me I found out I was pregnant with my first and only child. So supposedly you count how far along you are from the first day of your last period , but wouldn't that mean you are pregnant for that last period? If so is it possible I really am pregnant, I haven't taken a test yet because I'm not even scheduled to start my period till the 16th. I would  be 3 weeks along then by those calculations right? which is how far along I was when I found out with my son.