My period was due the 17th it came on the 18th the first two days it was just spotting than yesterday a normal flow what could iit be?

Likely normal : Periods are often irregular, either in start date or heaviness of particular day's flow. Minor stresses, illness, etc. Can also alter period. Keep a menstrual calendar so that you can better recognize what's normal for you. If you develop other, more worrisome symptoms--very heavy bleeding, nausea, faintness, severe abdominal pain, see a health care provider immediately for possible pregnancy comp.