Anitbiotics & birthcontrol. Pregnant?

I've been on birth control for 4 months now. Starting about 12am on Thursday morning I took Amoxicillen for strep throat. I had last night(that same Thursday just about 10pm) with my boyfriend not using a condom, he d inside of me on accident but quickly pulled out, although a small amount has still been inside of me. My last period ended less than a week ago. I'm just curious if there's a big possibility of me being pregnant from this even though I was on the antibiotics for less than a day and have steadily been taking the birth control for 4 months, also taking into consideration the small amount of sperm and earliness of the month for me. Please anything would help, we are planning to get the PlanB pill, if it seems we are at a high risk.