brown spotting question

af was due yesterday and did not come. had light cramps on and off during the day. today i didn't have much cramping, but i felt twinges on the left side. i had a pink/red spot in the morning and it's been brown ever since and only when i wipe. i don't believe it is implantation because the only time i had sex was a day before my fertile week started. i was wondering if anyone knew what this may be?