What Causes Brown Vaginal Discharge After Having Sex?

Hello. I am still a virgin. A day after my menstruation finished, my bf and I did something. He sticked his penis on my vagina but it didnt enter. I m afraid of the possibility of precum entering me. And just after 2 days when we did that, Im having brown discharge. What are the possibilities that I will get pregnant from what we did? Please. This is for my friend.