I read your post in the context that this temperature/heartbeat relationship is new, that is you have had experienced similar activity and ambient temperatures and during those times you did not have an elevated HB.
It is hard to react in any quantitative way not knowing the temperature, you level of activity or what your HR is. That said, discussing with your doctor is the best approach.
hi, one guess is that you undergo a large degree of vasodilation in response to heat - and then you undergo 'reflex tachycardia' as your system compensates for a big drop in BP. But that scenario wouldn't seem to apply to when you do simple physical activity.
Still, I'd get a home BP monitor and see what happens with your BP during these episodes.