Dehydration can certainly do it, as can the heat. Those two things often go together (you're more likely to be dehydrated if it's hot), especially if you're not used to the heat. My heart rate tends to be higher than usual when running or riding on a hot day as well, especially during the beginning of the summer (and it gets high enough anyway, so I don't particularly like the effects of a hot day!). Make sure you drink plenty of fluids; a good easy way to check your hydration status is to look at the color (and quantity) of your urine--it should be pale yellow. If it's brighter/darker yellow, you need to drink more.