Freaking out; tears and all

My fiancé and I were having sex with a condom and I’ve gotten back on the birth control pill; the condom got lost inside of me after he ejaculated. We pulled it out, and we both started panicking. We are not ready for children whatsoever, and he just ran out of the house to buy a plan b pill. If I’m taking it this early, what are my odds of actually becoming pregnant? I’m losing it.