We were making out, I entered a little bit in her, she was in "high fertility"

So, yesterday my girlfriend and I were fooling around. We were both naked and she gave me a blowjob and I was fingering her. We started griding and the head might have slipped in, were not sure if it did but whatever happened, felt different than before. Less than 3/4 of an inch was in. We immdediatly pulled out and stopped, it wasnt even in for a full second. I was not wearing a condom and her period app said she was at "high fertility" .. what are the chances? I did not cum before or after or pee, and I dont think I had any precum. Thank you