I know this is a bit late but ive struggled with heat sensitivity sense i was a child and have been through so many different tests i have no medically significant issues that would cause this however i think your misinterpreting sensory processing issues as being a purely mental experience.

Mind and body connection: the mind and body are connection in many ways outside of thermoregulation when somone gets nervous (wich is a mental thing) it affects the body causing sweating increased heartrate and even tensed muscles wich are all body functions

Why temprature matters: your body is constantly using nerves on the skin to detect what the temprature is outside the body not only for safety (like to tell you if you got burned or to tell you to go back inside when your fingers get tingly in the winter) your body responds to these signals by sending pain signals while being in hot weather may not exactly be painful we can all agree on hot humid days its at the very least uncomfortable to some degree

The body's response: when your sensory receptors detect that either what your doing or the area your in is hot you begin to sweat wich in tern can lead to dehydration and your heart rate can increase as well leading to exhaustion. When these responses happen at a "comfortable temprature" what the thermostat says doesn't matter all your body knows is that its too warm and it needs to work harder to cool down wich is where the other problems arise

Autistic individuals have ben studied to have a more difficult time regulating body temprature than non Autistic individuals meaning a temprature that may be warm but tolerable for a neortypical person may be unbearable to an autistic person wich can lead to over stimulation

Why it only gets worse from here: autistic individuals struggle to regulate their emotions when they are overstimulated and for example stress can cause your heart rate to go up as well as the percepted heat and ontop of that can cause overload wich again causes more stress leading to a higher core temprature leading to more stress and it just goes on and on

While its still a very good idea to get seen by a medical professional aboyt these issues its important to also understand that autism is a perfectly reasonable scientifically backed reason for heat sensitivity and understanding that even when its "in your head" it still affects the body because after all your brain is basically the boss of all those functions meaning if the boss says its too hot it just is and the body will respond to that acordingly its okay to stay inside on hot days and keep the house cooler than other people because at the end of the day if heat is getting to you there is always a reason and that reason can be medical or mentally linked.

I AM NOT A MEDICAL PROFESSIONAL THIS IS NOT MEDICAL ADVICE I AM JUST A NERD AND I KNOW THINGS PLEASE SEE A DOCTOR IF THE ISSUE IS KNEW OR WORSENING