[nigelH]

Sorry but that's a complete stop.
You diving will probably come back but you aren't going to risk traumatising some poor buddy who has to haul you out cold if that happens in the water.

Chase up your doctor and then take yourself to a full diving doctor who can do an exercise ECG, or send you off for one, and write you up to dive again.

That comes with the proviso that 'I am not a Medical Doctor' just a physicist who has had a heart attack and read it up from his wife's text books.
I got my dive medical back and I lost heart muscle so I think I was worse than you.

Good luck.

Oh and remember the pain. That's Angina. It's heart pain. It's different for different people but it is the stop-sit down, you're overdoing it flag. Some people mistake it for indigestion and just keep going. My father in law did. Took some rennies and dropped dead after dinner.