Will i get pregnant from a slight possibility cum touched the outside of my vagina or near the outside? he wore a condom and semen only slipped out the top in my butt crack after we had intercourse but nothing up inside. should i get plan b?

Not pregnant: Pregnancy is unlikely unless semen entered the vagina. Bear in mind that your risk of pregnancy from intercourse with a condom in a year is about 20%. If this risk is higher then you like, you should get on birth-control or a long-acting prevention method such as an IUD or implant that have a risk < 1%

Very low risk: ejaculate or pre-ejaculate would need to make it into the vagina or the introitus (opening of vagina). Chances are extremely low at the introitus.