Had to repost with the right dates.

I was having relationship issues and cheated (I know terrible). Pulled out and took a plan b pill (Jan 10th). Decided to work things out with SO and had sex the 12th (no protection and didn’t pull out). Had some egg white discharge the 10th. App says my most fertile days were the 12th and 13th. Everything has been good, then BAM late period and SO thinks I’m pregnant and buys me tests. They were positive. I know cervical mucus usually shows up 1-3 days before ovulation. I know the pill usually doesn’t protect against any sex after taking it. Whose do you think it is? The person I cheated with or SO’s?