If you spent a while taking in less than your recommended caloric intake your body might have taken it as a sign that resources are lacking and adapted, using up less calories than before daily and trying to store up fat to ensure a safety net if you reached the point of starvation.
It's not about your mass, it's about the quality of it. If you're working out you might have started putting on muscle, which is considerably denser than fat and thus heavier. You mentioned eating sufficient amounts, but you also have to consider the quality of your food. You should be getting plenty of protein and fiber (milk, lean meat and vegetables). Your carb intake has to be low ( think a slice of bread or a hand of granola with breakfast) and you should be avoiding fattier foods (no oil-if you fry something do it in a non-stick pan-, no butter, no bacon-lean pork is ok- or fatty cuts of meat or heavy dairy products-like cream). Cut down on sugar and replace it with fruit; avoid soda. Your food also has to be diverse: variation is key and will supply your body with the elements it dearly needs. Try finding food you like that fits the criteria: it'll make it easier to stick to a diet and just generally make life a little better.
Exercise will increase your metabolism, as will a higher protein intake. Make sure that you're getting enough rest-a fulfilled sleep schedule is important to weight loss. Make sure that you're getting enough water.