In 2011 I got heat exhaustion Ever since that I been sensitive to the heat. Why did that 2011 case seem to make me heat intolerant for life?

I am petite 36 years old. No MS. No thyroid issues. I  do have ADHD, Autism Spectrum Disorder, Sensory Processing Disorder. In 2011 I suffered heat exhaustion and dealt with headache, muscle pain, nausea, extreme weakness/fatigue. Did not even want walk 100 feet or less to kitchen.  I went to the ER Upon returning roommate/friend told me I had sun poisonig and recommended water and salt.  Was biking and protesting. Since then I have had heat intolerance. I do not like anything above 80. As a kid I loved heat. But since 2011 not so much. I feel tired, weak, wonky in the heat now. At work as a caregiver/personal support worker I can only go outside in the mornings with clients in the summer.   In 2020 got heat exhaustion again. Tired, dizzy felt faint. Nausea. I acted stupidity and sat back in the sun because I felt like if I moved I would pass out or fall. My friend helped me get to AC and gave me water. Put water on back of my neck like the Nicaraguans taught. Been more sensitive since then. Why has my sensitivity increased since 2011? I heard the 1st week after heat exhaustion is higher risk for re-occuuance. But I been sensitive ever since my first case in 2011. Why?