False positives are very rare. The second line only shows if there is hpt in your system. Pretty much the only reason for that is a developing embryo.
I got much stronger lines to begin with using SMU. I have no idea why.
Wait a few days, then try again. If you are pregnant, then the hpt will have had time to increase and so you will get a stronger second line. If the tests then ALL read negative, it is likely that you sadly had a chemical pregnancy (where the embryo begins to develop and releases enough hpt to get a faint line, but then stops developing very early).
If it's still unclear, you can go get a blood test from your doctor.