L.C.
It can be caused by any number of things like, weight loss, weight gain, stress, hormone imbalance. It could be a chemical pregnancy where an egg might have been fertilized but did not implant.
When my periods are late, they are always clottier (not sure that's a word) and heavier.
If it happens with the next period schedule an appointment with your OB/GYM to make sure everything is in sound working order, but right now I wouldn't worry to much.
It's crazy isn't it? No woman loves her period and would gladly skip the hassle, but let her cycle get messed up and it's a total mind blower.