BC and antibiotics

So tonight something happened and I'm worried. I'm on my break week of BC and I've been taking an antibiotic(Cephalexin) for some Cellulitis on my finger. Well tonight was the first I told my BF I love him and we made love and he ejaculated inside in the heat of the moment. Now we're worried and considering taking the plan B because neither of us want to be parents right now. I've had people tell me most antibiotics don't interfere with BC and other people tell me the opposite. Please help.