Negatives after positives

A

August 12th I took three pregnancy tests, all came back with faint lines, took another test yesterday negative, and as well as today it came back negative. Im really not sure why I’m still getting negatives. I was 9 DPO when I got my first three faint positives. Today I’m 12 DPO. No bleeding, no cramps I start my period 18th or the 19th. Has anyone ever had this happen to them? Or know what could potentially being happening?