HELP I MAYBE PREGNANT!!!

My boyfriend and I just tried anal for the first time. And he ejaculated in my bum. A little while after I noticed like if I used the bathroom or expelled gas of some sort the semen would come out. I was in the car a few hours later driving home and I naturally felt a moist sensation downstairs in my panties. When I arrived home I felt and looked and my whole kitty area was moist and full of a thicker liquid. I'm not sure if was some of the semen or just discharge (which I am known for doing). I told my boyfriend about it and he wants to buy me a plan B should I take it? Or am I being too over traumatic?