How possible is it?

So I know that I should be more cautious and careful during my fertile days. According to glow I have a 26.9% chance of pregnancy today. My fiancé and I are always extra careful during my high % days. I always have a normal cycle and check glow daily. Since I am not on and have never been on birth control (I'm 27 by the way). Today he came at the same time I was and I was on top. My body froze when he was telling me to get up, and so I stayed on top of him longer than I usually do when he says he's about to cum. When I was finally able to pull myself off of him and he came near my vagina, it was on my butt and on/around his dick. How likely is it that I could become pregnant from this. First scare in all the years I've been sexually active.

I know what you are all going to say, use protection, get on birth control, blah blah blah. I know. We are getting married Oct. 2018 and will be TTC soon after.

No rude comments please. I know exactly what happened and what could happen, just looking for a little advice.