Is this implantation bleeding or AF?

My last period began on November 21, 2010. I've never tracked my period, but i'm pretty sure it's a little more than 28 days. On December 15, 2010 I had very sharp pains on my right side so I think it was ovulation paints maybe? I don't know if I ovulated before, during, or after the paints. But anyway, I am now 12 DPO and this morning when I wiped there was a brown discharge on the toilet paper (more watery than chunky)...so I put a tampon in expecting that maybe it was the beginning of my period. Well I worked for 8 hours and when I got home I took the tampon out, and ABSOLUTELY NOTHING was on the tampon. Do you think this is implantation bleeding or AF? We've been TTC and have BD'd every day in the month of December except the 7th, 9th, and 14th.