Late Period, Negative Tests

It’s now the beginning of September and my last period was the first week of July. I have had almost the exact same cycle since I was 13, this has never happened before, the latest I’ve ever been was 1 week.

My fiancé and have not had sex, we are waiting until marriage, but we have done other sexual things. I’ve done my research and I know that it is extremely unlikely that anything we do could lead to pregnancy. However to rule it out I took a test two weeks after my missed period and two today: all negative. The only “symptom” I’ve had is swollen breasts.

I’ve had anxiety for years and stress has never messed with my cycle. I have a doctors appointment next week but I can’t help but worry about what might be going on because I can’t think of an explanation that’s good or makes sense. Please give me some advice and encouragement!