each test has a different threshold. there's a chance you are on the lower end of being able to pick it up (hence the faint positive)
It can also be affected by how much you drank prior to the test (and how long its been since you last peed)
if you're worried, then wait a day, first thing when you wake up, pee on a stick (best time of day to test) the "sooner" it claims to detect pregnancy, the lower the threshold is.