What Causes Brownish Spotting Without The Actual Periods?

Hi, I am 32 years old female who has been trying to conceive for the past 8 months. I used to have sex with my husband on all the fertile days . This month my period is due today (8/8/2011). Initially I noticed dark brown and scanty blood. But 6 hours later its becoming bright red,,but its quite scanty. I don't know what is happening to me. I don't have any discomfort at all in this cycle.. During my other periods I used to get severe back pain and fatigue.Please help me. Iam married for the past 8 months. Help me,,please