The water may have gone cloudy as a result of a bacterial bloom after the big rainfall. These bacteria use up a lot of oxygen. As the fish were gasping I would say depleted oxygen was the problem, thanks to the bloom. Adding a large airstone for a while would help too.
I have had problems with my pond in the past winter. Every time we had a heavy downpour, I lost fish. Water stayed clear though and no fish were gasping anywhere. Interestingly, I have spoken to several others around the country who reported the same thing. Big pond, heavy rainfall, sudden deaths. All had well established ponds and had never struck this problem before. The only thing we thought was maybe the very mild winter may have had something to do with it, allowing more bacteria (of the bad kind) and stuff to flourish when usually they would be killed off in the cold.