Today I exercised in a closed space and forgot to open the window during exercise. After completing 10 mins of cardio I felt very rapid breathing, light headed, darkness before my eye, very tired, sweating a lot and dry lips, felt I will faint. I opened windows as soon as I realized they are closed, slowly things came back to normal. I am generally active and used play football  until a week before. No history of any huge illness in past. This happened for the first time ever and I stay alone currently so I am worried 😞. Can someone advise if this could have been one time thing due to lack of oxygen? Thanks, Ashish