I honestly wouldn't worry about it. Sometimes periods just go away or come heavily. She might even be stressed out. Personally, I had normal-ish cycles for a while, they would venture into bizzaro land for a month (lasting 1 day or lasting two and half months) and then be completely normal again the next time.
I would wait until her next cycle until you decide to do anything. I would get her checked for PCOS immediately. I was diagnosed at 15 and I still went through a lot of pain and crap. If she has it, you want to control it before it becomes a problem.
But truthfully, it's probably some random anomaly-- who knows what our bodies are thinking? If its the same in a month and you still feel worried, get it checked out. Till, then, chill and relax-- no need to add more stress to I'm sure what is already a stressful life.
Sorry for the chill/Zen natureof my advice, I just have a nice 30 page paper due tomorrow and if I'm not calm, I'll be hysterical.
Cheers,
Diane
sundevil009@yahoo.com