A related discussion, Sun and heat intolerance was started.

I HAD TWO SEVERE HEAT STROKES WHILE WORKING FOR THE POST OFFICE IN AUGUST OF 2007. SINCE THEN I AM EXTREMELY SENSITIVE TO SUN AND HEAT. I LIVE IN TEXAS AND CANNOT GO OUTSIDE IN THE SUMMERTIME. I'M IN THE HOUSE 24/7 AND I HAVE 3 KIDS. I CANNOT WORK MUCH LESS GO TO THE CORNER STORE DUE TO THE TEXAS HEAT. I WAS DIAGNOSED WITH HYPOTHALAMIC DEREGULATION AND CANNOT GO OUT IN TEMPS GREATER THAN 78 DEGREES WITH NO SUNLIGHT. NOW THE DRS. AREN'T SO SURE OF THE DIAGNOSIS AND WANT TO CHECK MY AUTUNOMIC NERVOUS SYSTEM? DOES ANYONE HAVE ANY ADVISE FOR ME?  

Hi Cherican, Sorry about your symptoms. I have felt this way too but I was on a diaretic (water pill) which is known to cause sensitivity to the sun / heat. When I came off the diaretic, I felt much better while out in the sun. I gained a bunch of stamina back as well. I don't know if you are on this type of med or not but I think the drug was causing me to be dehydrated. So, whether or not you are on this type of drug, you really do need to stay hydrated as pointed out before. Hope this helps!

I'm not a doctor or health care professional.  Just an average joe.  I can just give you some ideas to speak with your doctor about, as I, too, have issues with heat intolerance.  Possible causes of heat intolerance, based on my research:

thyroid problems
parathyroid problems
hormone imbalance and/or menopause
neurological disorders
diabetes
sweating disorders

These are just some of the possibilities.  I am currently being evaluated for the heat intolerance, plus a number of other issues.  I highly recommend that you see a doctor ASAP to be screened appropriately for whatever may be causing your symptoms.  I live in an area of the country where it is routinely 100 degrees in the summer, and it's tough not being able to go outside.  I am being seen by a neurologist and an endocrinologist, after being referred by my primary care doctor.  We're going on 3 months now with no clear answers, but I am definitely hoping we'll know something soon.  What should you do?  See a doctor. ASAP. Only a doctor will be able to help you determine the cause, and possible treatment.  Or, at least be able to refer you to a doctor who can help you based on your health history.  I don't think self-diagnosis is the answer for you, see a doctor. KEEP IN MIND - this particular form "undiagnosed symptoms" is not moderated by a physician. I hope this is helpful to you.  I wish you luck in figuring out your issue.  Know that you are not alone.

My friend had heat stroke when she was 19 and passed out, she's been sensitive to the sun/heat ever since and gets sick as you described (& passed out a few more times). Do you have any autoimmune problems or symptoms? Autoimmunity (among other things) can make us a lot more light & heat sensitive. How are you abuot staying hydrated? (Make sure nothing you are drinking has artificial sweetners in it like Aspartame or Succrolose, that are often added to sports drinks and water.)