Sounds like you may have just been calorie deficient. Fat has more than twice as many calories per gram than carbs or protein, so if you're replacing carbs with fat, gram per gram, you're getting more calories.

I've been keeping track of what I eat to keep it within a certain calorie range. I generally try to maximize protein as much as I can, but the fat seems to take care of itself. From tracking food I've found that fruits and veggies, while good for you, have hardly any calories at all. Eat whole grains for carbs.