Pregnancy Test

Patient: Hello,My expected period was Nov 8 2011 and I suspected I may be pregnant but nothing came up on an HPT until 3 days ago. I have taken 3 First Response Tests since then, and the positive line is there but certainly not as dark as the other line. Should it not be increasing daily? I know I ovulated on Nov 28-29 so its not that I’m later than I thought. I am extremely worried because if it isnt getting any darker that I could be anticipating a miscarriage. Any insight would be greatly appreciated.