Happened to me a few times in life and every time for a different reason, although symptoms were the same, sweat, lightheartedness, dizziness, blacking out, weakness, and passed quickly with no consequences, although while it was going on I pretty much felt like I was having a near death experience.
First time I was about 12-13 and it was part emotional and part physical, it was really hot out, I wasn't drinking much water then, and just had a fight with a family member, and we were walking down the street in the heat and very angry at each other. I felt totally drained from it all, don't know which contributed more, but I suddenly had to sit on the ground and lean against the wall and was fighting unconsciousness. It never happened before so I thought I was dying and was scared. Paramedics came, checked me out, and said it was dehydration and also could be hormonal changes due to age.
Next it happened in my early 20's, I had bad eating habits then so attribute it to that, plus was also not drinking enough water but was taking caffeine pills at the time, and I think caffeine is not good for me and had something to do with that. I started taking better care of myself and it didn't happen again until a few years later. I passed out in a store, very weak, drenched in cold sweat, clammy, and felt all color gone from my face, blood pressure def plummeted. Was taken to a hospital, checked out, told it's dehydration, given IV fluid, and after a short while walked away fine. However, I drank enough water before it happened and the only thing was, I took some cold meds before leaving house that day, don't recall which, but guess it was something that should have been best taken when staying at home, and there might have been something in them that dehydrated me faster.
I also feel weak, faint and drained and cold sweaty in closed places with no natural air from outside and especially when there's a lot of people and I have to stand for a long time, like in line. Stores, post offices, etc. It just happened recently, I passed out in a public place like that, was drenched in sweat,paramedics didn't find anything wrong physically, blood sugar was fine, heart was fine, no dehydration even, I ate and drank enough before leaving house, so one of them said it could be social phobia or claustrophobia. I did feel better almost instantly as soon as I was taken outside and was in a fresh air.
So for anyone experiencing it there could be any reason and not necessarily physical. In my case I think I'm just very sensitive to energies and unnatural environments. Physically I always check out fine and recover fast as soon as I get out of a place where it happened. Gotta start carrying smelling salts like ladies in the olden days lol. Always a bottle of water and spray with some essential oils for sure.
Oh, also, don't know if it's a coincidence, but last time it happened I got my period the next day. In my 30's PMS symptoms have gotten really weird, sometimes there's none and sometimes something that used to never happen, like a migraine or this, so this could be a reason for someone else too, having to do with that.
To the above post, yep, I got the desire to go into deep sleep too. I think it's good and a healthy desire, what's needed for regeneration of the body and restoration of energy. When you are drained/low all kinds of activity, even just mental, will only take more out of you.