Why Am I Spotting Brown Blood For 3 Weeks?

3 months ago I tried the Hcg Diet and found that I could not do it. Currently I am taking phentermine and have found great results with it. I mention these things because my usually nonexistant period has been sparse and spotty on a consistant basis. Translation... I have been spotting brown ick to blood for the last 3 weeks. Should I be concerned?