Firstly, i honestly think a congrats are in order...

Yes, although the test's you are using may be sensitive, it doesnt mean it should make the line darker. All that means is it will pick up the HCG alot sooner than another test. So, if we go on that train of thought, that means the line is always going to be faint so early on... so i wouldnt be concerned about that. You may have also O'd later tha you thought, which would mean you may have only just had implantation, resulting in this very faint positive.

Id give it another day, do another test.... but going by all you have said i would say a congrats is in order

Let us know how you go