What Causes Headache With Fainting Feeling And Lightheadedness?

Whenever I lie down, a headache starts to slowly for, along with faintness. If I ve been lying down long enough, when I sit up my head will throb and it is the worst pain. Because of this, at night I fear that I m going to die in my sleep. When experiencing any other pain with the throbbing part of the headache, I get really lightheaded and almost pass out. This all started a week and a half ago with a tension headache that got super bad to where I could barely move my head from the muscles in my neck, shoulders, and part of my back were so knotted up and tight. I got in a pool for a few hours and the headache itself and the muscle thing we re almost cured. Now I fear passing out and dying whenever I m faint. When I wake up in the morning it takes a few hours for this faintness to pass. This is really terrifying so I d be overjoyed to know what could be wrong.