I know that whenever I was late and started worrying about being pregnant, it would come even later. Stress can make it late. I don't know if it's the same in your case, but maybe just try to relax and not think about it (easier said than done, I know).
Have you been tracking your periods? Cycles tend to change, they aren't always what they used to be. I'm no expert, but that's what I've heard. If you're consistently the same number of days late (or close to it), maybe that is just normal for you. Tracking is always a good idea in case your doctor wants to see the pattern.
A few years before my first son was born, I skipped a period. I went to my doctor for some tests and they said my hormone levels were off due to my recent weight loss (I had been dieting early that year). My cycle was so irregular for about a year, but one month it just felt different so I decided to take a test and sure enough I was pregnant.
I guess I'm saying it could be any number of reasons. But if you are really worried, try talking to your doctor. Good luck!