First, I am assuming you aren’t sexually active, since the most common reason for being late is being pregnant. So assuming you aren’t pregnant, it is possible that your cycle is undergoing a little change right now. Since your period last time lasted longer, it means either that you had a thicker lining to shed because your time between periods last month was longer, or that the ramp up in hormones to start the new cycle wasn’t as robust as usual. If you didn’t start your new cycle with a strong surge in estrogen, you would bleed a little longer, and it would then take longer in the next cycle for you to get to the hormone levels needed to ovulate. Some months pass without ovulation, and in those cases, your period may come later than expected, too. Either way, it sounds like this cycle was a little off in the hormone department and hopefully after your next reboot with your period, you’ll be back to your normal self. If you go for several cycles with things seeming off kilter, you should see your healthcare provider.
I don’t know if there’s any medical basis for it (Molly could answer that better than I can), but I’ve experienced the same thing. Usually, my periods are long and heavy, and I don’t have one every four weeks. But on the rare occasion that my period is shorter than normal, I’ll get my next one sooner — usually within the more typical 28 days.
My period is fairly consistent, yet its start date can vary by a day or two (being irregular is what’s regular for me). So, I’m not really sure of the answer to this…maybe our health expert Molly can provide a more definite answer.