4 days late

I was supposed to get my period Thursday the 16th. I am regular and my length is usually a 30-31 days on the dot. All my pregnancy tests have came back negative. Did get covid shot Aug 26, but I don’t know if that would cause my regular cycle to become irregular. Apps say ovulation was Sep 1/2. We did have sex August 28, 29, and September 4,5. Due to his job location we are only able to see each other on weekend, which puts a damper if I ovulate midweek. Any ideas why I’m late but testing negative still?