Is she on birth control? If she's taking the pill and not regularly enough, that can change her rhythm.
Is she fully grown? Oftentimes people don't fully mature until their 20s. Any change in hormone levels during a growth spurt can influence her pattern.
I'd say wait a few more days (3-4), then get two otc tests. If they say you're in the clear and she's not feeling nauseous or any other common symptoms of pregnancy, I'd say you're pretty safe.
Pregnancy symptoms:
http://www.pregnancy-info.net/pregnancysymptoms.html