What caused this?
I have worked construction for years and I can't count how many times I've worked out in the 100+ degree heat all day. I have never had anyone get serious dehydration. So, what I mean is it isn't easy to get dehydrated unless you are intentionally not drinking water. Or other reasons like being sick or drinking only alcohol etc.
If you are dehydrated for no apparent reason, then you should probably see a doctor.