We are all sorry for your loss. It is very hard to lose a pet especially after 13 years.
ANY guess as to a cause would be pure speculation, and we cannot do that here. I will say that
heatBabies and heat rashes
Heat emergencies
Heat rash
Infant heat rash
Tenosynovitis stroke is seen this time of year, heart attacks can be caused by heartworms and age was not in his favor.
If you get another dog, be sure it is on heartworm prevention early!!! and I would choose another method of flea
controlControl
Control rx (like Frontline or Advantage). Just my opinion.