Brown discharge instead of my period?

For the past three days I have had Brown discharge instead of my period which was due around this time. I'm also having mild cramping and some pain in my lower back. My boyfriend and I had unprotected sex on 5/21 its now ten days later. I'm on the birth control patch. Could implantation bleeding happen this soon? Can I be pregnant? Or is this just a freak period? Help would much be appreciated very soon!