I'm kind of in a weird situation here. My boyfriend and I had sex on Sunday May 13, around my time of ovulation. We used just a condom. We had sex again on Sunday May 20. I used spermicidal inserts and my boyfriend used a condom. I started my period one day late. It started normal, a bit lighter than usual, but nothing too different. I had some cramps, which is normal for me during the first and second day. The second day, usually my heaviest, was lighter than my first day. Today, the third day, is lighter yet. At this rate, I won't last my complete 6 or 7 days. The blood is not pinkish. It is a reddish brown. It looks like regular period blood. I don't know what is going on. Is this implantation bleeding or just a weird period? Should I be concerned? What should I do? So far I have had no other symptoms of pregnancy.

Just go to the Doctor for peace of mind. :)
It could be implantation bleeding or it could just be an irregular period.
Good luck

It sounds as though it is just a weird period. Mother nature throws us women for a loop every once in a while.

As far as implantation bleeding, please read this sticky.