What Does Bleeding Between Periods Mean?

Okay so I had my period about 2 weeks ago and I was at work today and I went to go use the bathroom and I noticed that when I wiped myself I was bleeding. It wasnt too heavy or too light, it was in between(if that makes sense). So it shocked me because i've never had an irregular period so it has me kind of worried. I did have unprotected sex like 2 days after my period was off, but he did pull out(i know you can get pregnant with pre-cum), but ive always did the pull-out method and never got prego. So anyways why am I bleeding 2 weeks before my period should come on if this has never happened to me before? Could this be an STD or implantation bleeding?