Plan B: confused on next period timing?

Hi! I was wondering if anyone else has experienced this. I got my last period July 22nd and it lasted until the 31st. I usually only have my period for about 6ish days. The 22nd I had unprotected sex and then got my period later that day. I took plan b the next day, and I think that's what made my period last longer than usual.

I usually use this app to track my period, and it says that I'm a few days late. I'm confused on if I should count from when the last day of my cycle was (31st) to this cycle, or should it come at the time it normally does? Ive been getting my usual pre-period cramps, I just don't get when my period of this month should be since I took plan b. Any info anyone has would be super appreciated!!!!