There are MANY reasons you could be late. I have been in your shoes before and I too am normally like clockwork when it comes to my period. Stress is a HUGE factor sometimes and even the stress of worrying about your period can cause it to be late.
Another possibility is that you are pregnant but your hcg levels aren't high enough to show a positive reading. While this doesn't happen to most people, it does happen to me whenever I get pregnant. I either have to have a blood test or wait.
Another possibility is that you are/were pregnant but there is something wrong. And I know it can be a touchy subject, but again, I have been there. If the baby stops developing very early, but you haven't physically had a miscarriage, it is possible for the test to be negative. This can also occur with an ectopic pregnancy.
Now, having said all that, you are most likely experiencing the first scenario, stress. Even though you may not feel stressed out, there could be something you are subconciously worried about. The other things that can cause your period to be off is significant diet or weight changes (gain or loss) or even if you have recently started hanging around someone new with a different period cycle-our bodies try to sync our periods with someone we spend alot of time with.
Hopefully you have heard back from your doctor by now, otherwise, good luck and don't worry too much. Everything happens for a reason, so no matter the outcome, it's all good :) God bless!