i would start out by drinking more. find a few drinks your body likes and try to drink a lot. lemonade, electrolyte sports drinks, water, salted water, soups, etc.
it also sounds like you may have contracted a parasite from the sushi. i am not vilifying sushi as i have eaten raw before for a number of years. but you might want to try some natural anti-parasitic herbs/foods. there is an herb called vidanga in ayurveda specifically for tapeworm. you can also try pumpkin seeds, pomegranite, pineapple, vinegar, jalapeno peppers, coconut, Clarkia herbal formula.
a lot of people with AF have trouble putting on weight. i would browse through posts here regarding paleo style eating. try to increase your animal fat intake.