The only way to know is to get a blood test or at home test. It's more common for false negatives than a false positives. So if you see a slight line you're most likely pregnant.
This exact same thing is happening to me as well. I had sex November 24 I did not calculate my fertile days but my period usually arrives the 16 of every month. Even thought it has only been 2 weeks I have taken three pregnancy test, the first two showed negative last one which I took yesterday December 6 showed a very light line of a positive sign. I'm actually already feeling pregnant which is insane. I am eating way too much than what I usually eat, I feel huge, I feel like if my ribs were expanding and my boobs hurt sooooo much that I have to sleep with smaller pillows on the side of my boobs & i tried to suck my belly in out of curiosity a few times i can no longer do that.