What Causes Delayed Periods With Spotting And Sore Beasts?

I spotted ( during sex, i'm not sure anything would have happened if i didn't ) but it wasn't even enough to fill up a pad. I even slept naked and nothing happened. I had sex on my most fertile days, all 3 of them. My boobs are pretty sore and i feel bloated and just plain uncomfortable. The spotting was mainly brownish-pinkish and definitely not normal red menstrual blood. Could it be implantation bleeding? I've read it appears just about a week before your expected period!!!