I'm probably overthinking this, but I'd really appreciate any help anyone could give me please!

My period finished on the 17th, and I had unprotected sex on the 20th. According to this app, my fertile period doesn't start until the 27th, but I'm worried that I could be pregnant?

This is completely out of character for me and I'm getting quite anxious about it, so any opinions/advice would be much appreciated please! Xx