Since your problems with the heat have gotten worse, I think it's worth a trip to your PCP to get checked out. I'm sure you're properly hydrating yourself, but an electrolyte could be off and causing the issue. Meds (especially cardiac meds) and too high/low blood pressure can effect how your body reacts to the heat as well. You're still going to be sensitive to the heat because you always have been, but hopefully you can get whatever's exacerbating the problem taken care of so you're not totally miserable all summer

When my heat issues got worse in my late teens, I found out I have hyponatremia. I've been on a high salt diet for a long time. I still have it to a lesser degree, but my blood pressure is stable and I haven't passed out in several years. My main problem now is due to the MS. Heat exacerbates my symptoms, the worst being the fatigue.