Super nauseous but not pregnant?

I am on birth control and just had my period. For the past week and a half by the time 5pm rolls around I have severe nausea until the morning. The only way to not throw up is to eat a very small meal which seems counter productive. I have also been experiencing quite a bit of lightheadedness after standing and I begin to walk. I have taken three negative at home pregnancy tests and haven't seen my man in over a month so it would show up if I was. What else could be going on?
Super nauseous but not pregnant?
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