2 and a half weeks late period

Okay so I had protected sex 3 weeks ago, I still haven't gotten my period and it's been 18 days 7 pregnancy tests, all negative. Took one today, negative. No signs or symptoms. I don't want to be pregnant but I'm scared. Am I stressing out too much about being pregnant? Do you think that by now I should just stop worrying and realize that my august period isn't going to come and I should just wait until the 21st of September to see if it comes then? Any help would be greatly appreciated