Negative to positive

So, today I took a pregnancy test that came out negative, or at least that's what I thought.  Now, I know there is only a certain window of time that you should check the test and not afterwards, but I went back and examined it again after about 10 minutes and found that there was a faint vertical line crossing over the horizontal implying a positive test.  The vertical line was even the same color as the horizontal one.  What does this mean?