Just Now I Had About 15 Seconds Of Unprotected Sex

Just now I had about 15 seconds of unprotected sex with a guy I've been seeing. We put it in and then pulled it out literally three thrusts later. I'm aware that precum can contain sperm, but know that the chances of pregnancy from this are way lower than if he had ejaculated in me. My question is this...does that 15 seconds or so of exposure warrent taking the Plan B pill? Or are the chances so slim that it's not worth it. Due to get my period any day now, have already had some spotting.