What Causes Insufficient Bleeding In Periods?

Hi, the first day of my last period was Jan 13th 2011. I had some light pink/brown spotting for 3 days about a week or so before my next period was due. It was suppose to hit on Feb 10th 2011, but I still have yet to have it, and it is now Feb 15th! So I completely missed it! I am trying to concieve, but all the tests keep coming up negative. Might it still be too soon to test? We want this to be our month so bad, but this has me a little confused. I would appreciate an answer asap! Thank you!