Help needed. pregnanct test dilemma xx?

Hi Guys. My period is due in a couple of days. me and my partner are trying for a baby so i thought i would try my luck and buy a pregnancy test. i got 2 in a pack as i thought i would try one this afternoon and another in the morning (excitement getting the better of me) I did what i needed to do and sat on the toilet in anticipation. Low and behold i saw two lines. One which was a little bit fainter than the other. i sat and looked at it for a couple of minutes before calling my partner. we were both overjoyed as we have been trying for 3-4 months now. about 10 minutes later I looked at the test and the line was fading out. it was barely visible but still there. another 5 minutes and the line had completely gone.I took the other test and it was negative? Can someone please advise on what could have happened? and if this has happened to anyone before as me and partner feel a little down now not knowing what is happening. Thanks in advance xxx