Hello, it's quite possible that the second test was negative because your urine was less concentrated - from what I understand a false positive is very rare. I would POAS again with first morning urine tomorrow for reassurance
Ditto what pistachio said (I do preg testing everyday as part of my job) and when I first tested myself I had a v.v. Feint line, I tested again a little bit later - nothing, I testing again the next morning - nothing!! But I am now 15w!!
Totally to do with how concentrated your urine is which is why the first urine of the day is best (assuming you haven't been drinking through the night)
Also, stupid question - what container did you collect your urine in? Some (very unlikely) containers e.g. Ones containing boric acid (they are given by the dr/hospital for urine collection for some tests) as they can give false results