Genetic Nausea?

So I have been suffering with daily nausea for about a year and a half now. I am 20 and this problem started back when I was 18. I get nausea ever day any time between 2:00 and 4:00 PM. I have been tested for everything my doctors could think of. My mom had the same problem when she was my age (not she actually vomited though, and I do not (primarily becuase I dont usually vomit anyways)). It lasted for almost a decade for her, and all her doctors ever told her was that she had bulemia but then they ruled that out and just said they didnt know. I am getting to that point with my doctors and all they keep saying is that it isnt genetic. Below are a list of my symptoms:

Nausea (largest symptom)

Leg pain (pain behind the knee, which is agonizing)

Fatigue

Now the medications:
Currently taking Promethazine for the nausea... which helps make it so I can at least eat, but only lasts a little while.

They have tried me on all of these with no success:
Zoloft, Zofran, Meclizine, Tylenol with codine, Melaxopam, Ativan, Maalox, Prilocect, Pepcid, and mussle relaxers

Any one with any answer or tip would be greatly apprecited. I am desperate for an answer