12 days! I got a better one for you...
Everything may well be okay. Your cycle can be altered by stress, anxiety etc. and then when it comes, it can be heavy. You may also have another issue. My cycles are well over 12 days (30+ days of light and heavy bleeding). After going to the doctor, I was told that I have small follicular cysts on my ovaries, BKA:PCOS (Polycystic Ovarian Syndrome), that's why my cycles were irregular. The bottom line is, if you're not in any pain wait until your next cycle and if it is like this one, GO SEE A DOCTOR!
Good luck :)