What is this? What causes this feeling?

It sounds a lot like anxiety... It will play a lot with your emotions and health. But unfortunately it could be something more serious and until you have a complete work up there is no way to be sure. Although once you go to the doctor and get cleared you can than cope with it being anxiety. If the symptoms get worse or you continue to faint then take yourself to the ER. Hope this helps and hope you feel better.