I feel like my brain is swelling in this heat?

It isn't really: The brain is highly protected from temperature extremes. But stay outside in high heat without drinking fluids for a long time, let your core temperature reach 104-105°f (this is called heat stroke), and your brain will swell. But you'll probably be comatose or nearly so, so you won't really notice. Some mammals have evolved more heat-tolerant nervous systems, e.g. The eland and the oryx.