Guy A or Guy B

Okay I just found out in pregnant yesterday (10/1) with a VERY strong positive. I had sex with guy A on 9/14 and 9/17. I then had sex with guy B on 9/23.. currently freaking out about who this baby belongs to!! After having sex on 9/23 I took a plan b pill on 9/25. Is it likely that I was already pregnant before sleeping with guy B? The first day of my last period was 9/1.. so when I put that in it says I'm 4w4d today but how could that be if my last period was 9/1? I'm very confused about how all of this works and I'm very ashamed I'm the girl in this situation because I have only slept with one man in the past 3 years so naturally when I sleep with someone else this would happen so I can worry myself sick!! Someone please help me! The line on my test showed up instantly as soon as the pee was running up the test there was a line before it could even get to the control line.