Late, short period???

Has anyone ever had a short light period and come to find out you were pregnant? My period was 4 days late this month and has only lasted 3 days when it always lasts 5. My period has always been super heavy but this month it's very light. It started out to be spotting, and then a normal flow(not heavy like it always is) and then It turned into brown spotting and then stopped. I've been feeling off this month, so I'm not sure what's going on. I took a test the day of my last bleeding and it was negative. Not sure if I'm just having an off cycle for the first time or what it is.