What Causes Sudden Dizziness, Sweating, Weak Knees And Fatigue While In Public?

For the past 3 to 4 years when I go out in public, I often get sick. I get queasy, light headed, sweaty, weak in the knees, lose my energy and I can t stand still. I have lost my vision. Two times I passed out and ended up in the emergency room but the doctors did not know what was wrong with me. Several times I got sick in different doctors offices but they did not know what was wrong. I am afraid to go out anymore. Elaine