Every Now And Then I Get This Feeling In My

Every now and then I get this feeling in my body that everything just drops like gravity is pulling at me and I don't want to move at all. I get really shaking and feel as if I'm going to pass out. This doesn't normally occur back to back just every once a few months until recently. I had two of these occurrences a few days apart. We don't know what it is and would like to. When I get this feeling and start shaking it all lasts for a few minutes. When this does happen I feel really weak and like a can barley move and it's hard for me to stand.