Ditto what @PatMattos says. I have way more problems in the heat and try to stay in the AC when possible. Heat and humidity are not my friends.

If you notice the emergence or worsening of a symptom like fatigue, labored breathing or a drop in exercise tolerance go in and get an EKG. Stay hydrated and avoid peak heat. I live in Arizona and hikers dying is routine, even fit ones.

Listen to the symptoms and by all means take them seriously.
Do you have any known issues with your heart or are you concerned that you may be developing one?

I have hypertrophic cardiomyopathy, so I do know why it happens and I just have to take it easy.

Hopefully you are being fully treated. Has your doctor suggested an exercise program? Might be a little hard to manage in warmer weather so maybe you could do something indoors. Of course ask your doctor first.

http://www.ncbi.nlm.nih.gov/pubmed/23928567
...
CONCLUSIONS:

The present study suggests that patients with HCM who remain symptomatic despite medical therapy may achieve considerable functional improvement through a supervised Exercise Training program.