What Causes Dark Brown Spotting During Menstrual Period?

Hi, I went off the birth control pill in February, got a normal period on 2/12/11 and another normal period on 3/21/11. On 4/21 I had very light brown spotting for 8 days. Yesterday (5/7) I had very light brown spotting again and this morning I had no blood but some dark brown tissue accompanied with very light brown spotting very briefly and this afternoon there is nothing. All HPT's have come back negative. Is this a light period? I am trying to conceive so it's difficult to figure out when I am ovulating. Thanks.