What Causes Chronic Nausea?

Hello, I feel constantly sick, its worse when io eat, stand up or any kind of movement, I am also normally sick about once every 2 to 3 days because i cant control it, it just comes up. i feel the sensation go away for a few seconds when i force myself to swallow saliva and take a couple of deep breathes but as i said it is only for a few seconds. I have been feeling like this for a few months now so any help would be great. Thanks you Harlan