This could have been me a few weeks ago! (except we aren't trying and don't want to be pregnant again) We had an oops moment and had sex a few times right before and while I was ovulating. What you described, is exactly what happened to me! I did not end up being pregnant. I was convinced I was, because my periods have been the exact same since I started having a period. Turns out, despite my regularity...it was just a really weird period. I had the bleeding like you, for 4 days before a normal type of flow started. I even called my Dr. She said implantation bleeding, is not very common. It is usually just drops once or twice and hardly noticeable. She said, it's pretty common to get weird periods when you have frequent sex right around ovulation.
Your best bet is take a test on the 17th, first thing in the morning. Good luck!