What Causes Weakness, Lightheadedness And Excessive Sweating?

I have been feeling very faint and just overall weak for the past four days. I thought it was due to a lack of sleep, food, water, or sugar but now I know that can t be the reason. I get very lightheaded at certain times with no set pattern and I sometimes get it bad to the point where my legs are all weak and my head feels so ...airy. I m not sure why this is happening but last year, i had a sudden episode where I got very very lightheaded and had to sit down for about 15-20 minutes for fear of falling. I also began sweating sooo profusely, my shirt was soaked. I am a 25 year old healthy woman, at 105 pounds and approximately 5 4 . I d really appreciate help.