Depends on how carefully you measure the food that you eat, how well you know what is in that food, and how faithfully you report all that you eat to a food log so you get a very accurate picture of what your caloric intake is.
Don't even try to guess offhand how many calories you expend in physical activity. It's difficult to be extremely accurate about that, nowhere near as accurate as knowing how many calories are in a bag of frozen berries that weigh X amount of ounces and have nutritional information on the package.
So, if you are very accurate about the measures of what goes in your mouth, then the estimate that you only eat 300-500 calories a day is way out of whack, requiring you to see a doctor. Even if you are 5 feet 4 inches tall.
Fitday (and other websites) have tools for you to put in your height and weight and activity level (underestimate, rather than overestimate, that level - to start, then you can tweak it much later on if necessary). You can find how many calories you need to maintain your current weight.
Two pounds of weight is not much. For some people, that's actually water weight when they eat a lot of salty food - and it goes away when they watch the salt in their diet. It goes away within days.
If none of the above are factors, then a frank discussion with your doctor is in order. Eat only all the food from bags (frozen food) or boxes (all the contents) and bring the bags and boxes as proof. The nutrition information is right on the bags and boxes. Show your doctor, and give the exact time period you cover eating those foods. Weigh in at the beginning of that time period, preferably right there at the doctor's office. Then weigh in at the time of the visit to discuss your problem. He can see the severity of your condition and put your mind at ease with information he can show you when he has results from blood tests.