Period VS. Implantation Bleeding

So in October I got off the pill which I had been on for about six years. I've since gotten a period every month, which lasts seven days. This month was different. I stared spotting the day before, which isn't too uncommon for me. Then I got my period but it only lasted three days. It wasn't very heavy either; taking my tampons out hurt (Sorry for tmi). Today I'm spotting but barely. My coworker mentioned it might be implantation bleeding. I took a pregnancy test but it's negative. Any advice?