First of all a gain in muscle mass means that your body will burn more calories just sitting there than it used to exercising before you got in shape. Secondly 2500 calories is a little high for someone 150lbs. For women it is usually in the area of 1800-2100. And if you have been eat ing WAY less than this maybe you aren't eating enough! Your body needs a certain amount of calories to survive and function, and if you aren't eating enough then your body will hold onto any calories that it gets with a death grip because it thinks it's starving.
Go see your doctor, and get the REAL numbers for you personally, and follow what she says!
The first and the best victory is to conquer self. - Plato