Babies use a lot of your nutrients to grow, therefore I would suggest you eat a well balanced healthy diet. You should be gaining weight, please ask your ob-gyn how much you should personally gain each week.
I agree, you need to see your Ob-Gyn. You should be taking prenatal vitamins and eating healthy food in a balanced way, with meat, poultry, maybe certain fish that are not high in mercury, vegetables, fruits, and healthy starches (for the calories and fiber). Don't use alcohol, drugs, or even much soda pop as those are worthless calories.