Implantation bleeding or Plan b spotting?

So i had unprotected sex on 9/1 and took plan b within 2 hours. I am not overweight or whatsoever so i supposed that plan b should be very affective. I checked the app flo and it said that my fertile window was supposed to start 9/3 but this app says 9/1. Anyways, on 9/6 i saw that there were spotting, which then turned into something heavier ( i had to change pads consistently) today is my third day and is still needs for a pad but the blood flow is getting smaller and smaller. I’m too young to have a baby right now and this is freaking me out. When does implantation occur? would 5 days after sex be too soon? and can anyone with plan b spotting and or implantation share some of their experience and symptoms that they went through so i can further more determine myself. Thankyou so much guys