About 9 months ago I was on a train standing up and I suddenly began feeling very hot, sweaty and nauseous and my vision began to black out, however when I got of the train and sat down it all began to fade away and I was just left with a headache after. I thought it was because I hadn’t eaten breakfast that morning but I regularly don’t and that has never happened before. Then today I was in an assembly hall, I wasn’t properly sat down but I was perched on a radiator and it was quite hot but for the first hour I was in there I felt a little bit sick but I was fine and then about 1h15min later I suddenly felt really hot and nauseous and my vision was blacking and my ears ringing, so I left sat down and it cleared but whenever I stood up again it came back until after about 10 minutes and I’m left with a headache again. Anyone know what’s caused this/ what it is? I’m a 16 year old female with perfect health.