Being ill or having an infection can make your period late. It's common. So can stress, losing or gaining weight, changes in routine. So, it could just be that. Hormonal imbalance even if temporary for a month can cause it too. I see you say you have no real reason for being late but this is also not common as it can happen out of nowhere. If you are having unprotected sex, pregnancy is always a possibility. So, you are smart to test. They are accurate but you are right again, you can get a false negative if you test at the wrong time. And if you are irregular for a cycle, it's hard to know when the right time to test is. I'd wait one full week, not worry about this and retest if it is has not started yet. After that, you can see your doctor. Most docs want you to have a period at least ever three months. It's frustrating, I know. My guess is you are not pregnant but having a really off cycle. good luck