Please Help- High School Pregnancy Scare

Last night- the night before the day my period is expected to come- I was fooling around with my boyfriend and he ended up putting it inside me with nothing on. He did not finish inside me and only briefly penetrated me but I am worried about whether or not there was precum. Today is the day I’m supposed to get my period, and usually I wake up with it, but right now it’s noon and still nothing. I’m very worried, should I buy Plan B to be safe or give it some time?