<s>[INST] Hi I m a 20 year old female. About 2-3 years ago I started getting very lightheaded and it s constant. Everything goes black my head feels like it s pounding but also shaking and my knees start popping like they are about to collapse on me. I have collapsed twice every other time I stand there. I can t hear everything that is going on and I can talk to you while it s happening. I am a smoker I usually smoke a half a pack or a full pack a day. After the lightheadedness I can feel the veins on my forehead and in my temples pulsing. Is this something serious should I be worried? [/INST] Hi,    I have read your query & understood your concern.    From your symptoms I feel possibility of following conditions causing your problem-      -Severe anemia-with history of fatigue,pale skin,brittle nails.      - Hyper ventillation -due to stress/anxiety.      - hypoglycemia-due to weakness following low blood sugar'       - Swelling of middle ear-associated with vertigo.    To differentiate the  cause you need to visit a doctor for examination/investigations,to be followed by specific advised treatment. treatment.       Thanks. </s>