Are you sure the line wasn't there before you took the test? could be just an abnormality on the paper that was there before that you only NOTICED afterward because you were staring at the window for a result. In my experience the lines get DARKER over the next few minutes, not lighter. The fact that it seemed to wash away as your urine passed through would make me think it was something already on the paper, not related to pregnancy hormones.
The reactive area is ONLY printed in ONE line. If a horizontal line really did appear when you tested, I'd say clearly, there was some malfunction in the manufacturing on your test. If the RIGHT strip got put in crooked, the line would appear facing the wrong direction with the same chemical reaction as if it had been facing the right direction (in which case it's a positive). But if THAT went wrong, I guess it could have been almost anything.
Test again tomorrow.