So having to poop can happen for a few reasons.One way to help is to alter the time of your runs or meals.
But also. It could be your body trying to release heat. One technique that helps me is to start my run with shirt that's cold either from the freezer, from cold water. Or both. Buys you a little bit of time without melting in the summer heat.
This last part isn't any sort of commentary on veganism. But for the hot summer months if I eat very light, cold meals with not much meat oil cream or sugar it's easier for me to deal with the heat. Might help you.
Running early in the morning could help too It's cooler.