It isn't weird at all, and yes, you can have a heart attack when your potassium gets badly out of whack. 3.5 is borderline...it's just under normal. 3.0 is bad (particularly combined with indicators of extreme dehydration and poor kidney function).
Moral of the story: Drink your fluids. Sip, Sip, Sip. All day. Everyday. When you are at more risk for dehydration, you have to extra careful with overheating in ways you weren't before.
I remember being able to chug almost a quart of water when I got overheated before surgery. These days, over three years out, I can barely "chug" 6 ounces.
Learning to drink water all the freaking time when it's hot...is essential. Eating your bananas and salty stuff is a good idea, too.