Hi with my DS I did a clear blue test when I was 2 days late and the line was very faint, we were unsure. So did the one which gives you a pink dot twice and they went pink. I think it said that the later you are the fainter the line, or something like. Don't know if that helps!
thought id add my two penneth- if your ttc and regulary buying tests try ebay, i bought 3 tests on there for about £1.20 and they are the most sensitive you can get they measure 10MIU,(most shop brands ie clearblue etc test 20/25/50MIU)
I did a clearblue about a week before my af was due and got which I thought was a line but really had to strain my eyes and thought I was imagining it but then 4 days later did another one and there was a definite dark line, so keeping everything crossed for you. I cheated on the first test and took the test apart and saw the line very clearly, perhaps you could do that and done one a few days later. Mine came through for me so perhaps yours will too.