Based on the information you provide, the day you discovered you had your period (i.e. the halachic day that commenced after shekiah – sunset) counts as the first day for the purposes of the five-day minimum and for calculating onot perishah.

This cramp could make a difference in the calculation of the five-day minimum if it is a veset haguf. That would be the case if this type of cramp only comes with menstruation, if menstruation follows within a time period shorter than that between the onset of this cramping and shekiah, and if you therefore abstain from the onset of such cramping.

If you have further questions or details to share, please write back.