Increased heart rate is a sx of dehydration although if I remember yours runs a bit high under stress and anxiety too. Drinking 8-10 glasses of water per day is the usual recommendation and don't use thirst as an indicator that you should be drinking. On those warmer days take plenty of breaks to get into the shade and drink your water.
Having your child leave home and go off to school is bound to cause you some anxiety. It is hard not to call your son but I am sure he is only trying to be independent and not upset with you. Just remember to let go of the hurt feelings and know all will work out in the end.
Kindly,
Kitt