Did you try testing first thing in the morning?
The first test I took was a very faint line so I went and got a 2 pack and took another test the same day and it was negative so I waited till the next morning and tested again and it was positive.
The first test i took was faint positive, then the next 15 were negative... i was pregnant though.
I've heard of people getting a positive, then a negative due to a chemical pregnancy? I had a friend who went through it, BUT obviously I can't say that's what it is.. that's just the only thing that I personally know of that's happened.. I would probably go to the doctor just to make sure!
do you know about how far along you would be??