I have had two cycles where this happened. Could be that your body is having a bit of an issue ovulating so is pushing the hormones, could be that you tested just as you stated to surge, tested when you surged and tested as you were done surging. And, if your FSH is up, that could be causing a false positive.
But, the other signs are really good so don't worry about it but get to the BD-ing!
And, don't jump to the idea that your tubes are blocked but an HGS might be a great idea and will probably be recommend by the Dr.s.