What Causes Nausea And Headache After Each Meal?

I am a 33 year old female who has been experiencing nausea after any food intake, headache, weight loss (20 lbs in 2 months) for approximately 3 months. I am 6 ft tall and weigh 126 lbs. I am constantly hungry but get extremely nauseated and crampy any time I eat. I know that I have an enlarged thyroid, but my levels always come back normal...I am getting tired of the continuous remarks from others that I am a stick, or that I look ill...and I have a fine tremor in my hands, which i have always had, but did not know if that might be pertinent. Help!