implantation or no?

Hi everybody, I had a question. I'm probably being paranoid... However my boyfriend has unprotected sex and we just use the "pull out" method for birth control. I was on a plane coming back home yesterday, and it's been 4 days before my expected period. And it looked as though I had spotting just a tiny bit (a few specks on the panties and a little pink discharge on a tissue when wiping)... It was a pinkish color, and I had a little bit of cramping. However, I'm not bleeding or cramping anymore today... Is it possible it could be what they call implantation bleeding? I took a pregnancy test right after though in the AM, and it showed up negative. I don't have any other symptoms, but I've been a little emotional, which could be pms... It bothers me because I never have spotting.