Yesterday as I was working out around 11 pm I was on the treadmill for a cool down after working out with some weights. I set the initial speed to 3.0 mph and heart rate was around 110 so i lowered the speed to 2.5. Meanwhile my hands are on the sensors to check my heart rate. As soon as I put it to 2.5 mph my HR skyrocketed to 140 and all of sudden my chest felt sunken in and was losing vision . But I got off right away and went to the front desk for assistance. He set me down and I drank 3 bottles of water and as I was drinking, I was shaking uncontrollably for 30 mins. But prior to working out I was working at my job from 2-9 in the heat most of the day and was sweating a lot. I drank a good amount of fluids and was peeing quite regularly. While I was at the gym though, I did not produce any sweat at all. I am fine today but still feel a little dizzy.