What Causes Pink Blood Discharge From Vagina?

Hello, My Boyfriend And I Had Unprotected Sex For The First Time On 03/27/10 And Since Then We Have Been Trying To Have A Baby. On 04/05/10 I Had Some Brownish Spotting That Lasted For Only Three Days. When I Had The Spotting It Was When I Was Supposed To Have My Period. I Have Also Took Many Home Pregnancy Test And They All Came Out Negative. Then Just Yesterday I Went To The Bathroom And When I Wiped A Light Pinkish Blood Came Out. This Morning It Still Was Light Compared To My Period But It Turned Into A Brownish Color And Now I Have Some Cramps. What Does This Mean? Can I Be Pregnant? Thanks.