It's possible to get an odd ovulation or delayed ovulation even in a cycle that is regular 99.9% of the time. If you have stress, take aspirin or nsaids close to ovulating, if you've had something to eat that effects your hormone output of estrogen (ex soy can cause you to ovulate earlier on average), medical intervention, sickness (especially fevers), low vitamin D. All these can delay ovulation or make it come earlier than normal and therefore make your period earlier or later.

Before jumping on the shedding wagon which is still possible [i don't rule that out) maybe investigate the more mundane stuff that happened to her between days 11 and 16 of her cycle.

Also; if you gear up to ovulate and one of the above delays it...then your body has to gear up again and this can cause a thicker than usual lining to develop which will cause a heavier period or possibly what seems like an early period because you get breakthrough bleeding. Expertise: 10 years of ivf and trying to conceive. I've seen it all and had it all. Nsaids for migraines were the worst disrupted of the ovulation cycle for me. Quit those ASAP if you can.