plan B after period

Ta
So I just had sex and he didn't pull out in time and I'm not on birth control. I go to my gyno Thursday to get back on it. But I need to take the plan b pill and I was just wondering if it would mess me up because I just got off my period yesterday and I haven't had one in a year due to nexplanon and I got it taken out in June. I'm really stressing out because 1. I hate pills and I don't want it to kill me or give me an allergic reaction because I've never taken it. 2. I don't want to get my period again. Could someone please help 😩😩