spotting between periods or implantation?

Hey guys, so around April 15 my fiancé and I had ancondom break and it looked like he came inside me so I took a plan b. I was due for my period around the 23rd-25th of April (I'm irregular mostly) my period came on the 20th and I was not surprised it came early since plan b normally initiates my period a few days later anyway. Now it's May 10th and I'm having either an extremely light period or spotting (can't really tell?) the blood is dark brown so it's old, but I'm not due for a period in roughly 2 weeks. I know plan b messes with your hormones and causes weird period fluctuations and spotting, but I thought it would be out of my system now right? I took the pill April 15th and now it's may 10th so almost a month! I highly doubt I'm pregnant because I did have a very light period brought on by the plan b, but I am still concerned?