What Could Cause Change In The HPT Result?

I have been feeling nauseous on and off for the past few weeks, but during the past week it has gotten worse and more frequent. Recently my breasts have become sore to the touch. I bought a two pack pregnancy test, and as I mentioned in the question, I took the first one and it came out negative so just to be sure I took the second one and it came back with a very faint line. I'm curious as to why they both have different results. Could it be the amount of urine that I put on the tests? The first one I urined a generous amount which is the one that came back negative. On the second one I urined in a cup and dipped the test in it and took it right back out and that's when I got the faint line? Could too much urine cause the test not to work properly?