Heat issues

I have always enjoyed being outside year-round and the weather has never bothered me. I am 18 weeks pregnant and I absolutely cannot tolerate the heat. I have been out of morning sickness for about a month now but the heat is still extremely hard on me. Literally, if it's above 75 degrees I become extremely uncomfortable- headache, nausea, diarrhea, terrible muscle cramps, and dizziness. I have tried what seems like everything to try and tolerate the heat and humidity better- sitting in shade with a fan and drinking plenty of water. Even through my efforts, I've still ended up in the hospital with severe dehydration and heat exhaustion. Is there anything else I can do while outside that can make this better? Has anyone else experienced this? My doctor told me to not let myself get uncomfortable because if I feel bad the baby becomes stressed as well. However, my job sometimes requires me to be outside this time of year so I'm trying to figure out a way to still be able to do my job and take care of myself and my baby. Any suggestions?