possible?

Je

I had sex when the app said 2.7% chance, but it's not actually showing that I was in the fertile period. I know that the app only guesses, but I'm a few days away from starting and my back hurts more than it ever did with period cramps. Since my tubal my cramps are mostly in my lower back. My right side where I'm missing my tube is constantly cramped. I feel like I could start at any second, but what are the chances? I'm thinking NO WAY!!!