So i purchased a set of Continental Grand Prix 4 seasons about 500 miles ago, and have had 2 puncture flats so far. Both rear tire. Previously i had on thickslicks, without a single flat. The first puncture was a shard of glass. The second puncture was either a piece of a branch of a nut shell, hard to tell, but it punctured the tire and through the top and bottom of the tube. Keep in mind, im a clyde. im about 240lbs right now. Also, i keep the rear tire pressure at 115lbs. Is it just bad luck or do i need a more puncture resistant tire? Id like to stay with a lighter weight tire if possible if i need to make a change.