I'm never using one of the Clearblue tests again (unless it's a digital) as I got that lovely "skinny" little line on the window recently (well, over a week ago) and had stupidly gotten my hopes up that I was indeed part of the BFP club.
I've read on other forums/via friends/research that the CB tests are REALLY prone to showing this little "Evap" line - even very quickly afterwards - and showing a "false positive" of sorts.
Get another test - a pink dye one (even the cheapest ones are pink dye tests) and see how that goes. Fingers crossed for you that it's a + though!
(Sorry, was really angry at the CB test for this...and then to hear that it was a "known issue" made it worse. A thick line tends to still show a positive but I'm staying clear. I can't be doing with that kind of emotional upheaval - nor can anyone, I think)