You should see your obstetrician for evaluation.
Keeping a log of your diet, including beverages, and an activity log can be very helpful to your doctor in pinpointing problem areas can be very helpful.
An over the counter home pregnancy test is advisable now if you have normal cycles.
You should use contraception to avoid further pregnancy until you are ready for a baby.
Not having regular cycles can be quite dangerous, leading to uterine cancer, infertility, and even death. Although skipping cycles seems minor to most patients, it is quite serious and should be treated.
It may be reduced with daily vitamin E or oral contraceptives.
A home pregnancy test in 10- 14 days is in order. Regular contraception is also advisable if not contraindicated by a medical condition.
If no, better she should see a gynecologist.