Making 2nd nut flush with a single Spade in your hand is dodgy as someone could and in this case did have the bare Ace of Spades, I was in a similar situation playing King/Ten suited and hit the flop right away making my flush, as the hand progressed a fourth card appeared on the table and I then became a little concerned as to this possible outcome.
As I was first to act I put out bet after bet after the flop and turn feeling I had the hand in the bag, but when the forth card came I was cautious, betting about a third of the pot on the River, Sure enough when the betting came to the last player in the hand he shoved all-in and I instantly felt he had a bare Ace.
I reluctantly folded the King Flush and another player in the hand called with the Queen flush only to be busted out of the game, and when the cards were turned over I showed my folded cards to the table saying I made the right fold.
No-one could believe I had folded the 2nd nut flush, but to me it was the right thing to do and was proven correct.
When the fun Stops, STOP!