Yesterday I was on the train to Paddington (London). At first everything was alright, I was daydreaming and thinking about daily life situations. I was standing next to the door and looking through the window. All of a sudden I get this feeling of dizzyness/blurred vision like I'm seeing stars on earth, then I couldn't breathe properly and I was sweating more than when I workout in the gym. I thought that was it, I'm gonna collapse and die, but I held on until the train stopped and the doors opened and as soon as I took my first breath of fresh air. That breath of air felt like being dehydrated and having your first sip of water in days.
Can someone please explain what this is? It's happened to me before it's not the first time.