If you have a very faint positive pregnancy test, then the chances are pretty good that you are indeed pregnant.
However, the likelihood, in my own experience, changes with the kind of test you're using.
If you have a very faint positive on a test that uses blue dye, like ClearBlue and most store brands, then I would absolutely not consider that positive until you confirm either with a digital test or a pink dye test from a brand like First Response.
For reasons I don't fully understand, you're much more likely to get a false positive on a blue dye test than any other.
If I were you, I would take another test, either way, two days following when you got your very faint positive. If the line is darker, then that's a good sign that you're definitely pregnant.