Talk to Gynecologist on Early Periods/Spotting Instead Of Periods.

I'm married and have two kids. My periods are never late. Last month, my period was expected on 23rd March instead I started spotting (light bleeding) on 18th March and it continued for few days. I did a strip pregnancy test multiple times, even two days ago, each time it was negative. This month my periods should start at 18th April but it is 11th April today and I have started spotting again. What could possibly be the reason?