I am 78 and on a hot day i had a cold fruit drink that was about 80% crushed ice. It wasn't cheap, so I ate more than half of this "drink".

[Naoris]

I went out and walked for 5 minutes and then I must have fainted, because the next I knew , I was in the back of an ambulance, ending up in hospital. 24hour testing showed no cause, and my heart has been perfect for the past six weeks.

A few weeks later, my "shoulder" Doctor thought the ice affected the vagus nerve in my stomach, lowering my blood pressure and hence I fainted.

[SyncopeTrust-STARS]

Your doctor was right in his diagnosis. You have experienced a type of situational syncope through drinking a very cold drink when you are very hot. This will have caused a drop in blood pressure and lowered heart rate in response to the drink. For some individuals this response can lead to fainting. I hope you have not experienced further episodes but I suggest that in the future you sip ice cold drinks until your body has adjusted to the change in temperature!