Two day? Period or implantation?

I was due to start my period on March 20. It started on the 19. Well, what I assumed was my period. It lasted three days and was fairly heavy. On the second full day I had awful cramping my my bladder that subsided and only continued on the left side. Last night it just stopped and started spotting earlier today. It's only been spotting now. No more bleeding? What is going on?