What causes nausea?

OK so for like the past few months I have been extremely nausea all the time. And I have gone to the doc they say I'm fine but if everything is fine then why an I nausea then something has to be wrong. I get it to the point of wear I have to go lay down and curl in a ball crying and if someone even sits down on the bed it feels like an earthquake. They have given me all kinds of nausea medicine And nothing seems to work.I even took a pregnancy test months ago to make sure I was not prego . SO what causes nausea? and any tricks to make it go away
What causes nausea?
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