Keep an eye on your food intake. As a biological entity, we are programmed to overeat (well the vast majority of us are anyway) without thinking we overeat in order to store food in case of a famine or we cannot get anything to eat. It predates to when we were cave animals and hunted for prey.
Try to work out how many calories you need to get to your goal, weather it be maintain, gain or lose weight. If your trying to lose weight, you will be burning more then your body requires so you will feel tierd, its natural. The website has dozens of stickes to help you with this.
Sleep matters but also be aware if your starting anything new or making changes which are quite sudden, your going to get side effects like fatigue. If you didn't you'd be able to do more and it wouldn't be a challenge. Everyone would be able to do it, it wouldn't feel like such an achievement.
In terms of low energy which continues for more then a week or two, go see a doctor, get a blood test, make sure your not missing out on anything. Could be easily sorted but its rare that fatigue like that would come on all of a sudden, tends to be more of a gradual thing.
Try to eat a balanced diet anyway (saves you the blood test!) eat a range of differently coloured fresh fruit and veg, lean protein (1.5g per 1kg of body weight, so 120g of tuna has around 30g of protein in for an example). Carbs, stick to low GI stuff like multigrain, wholegrain etc (not white, not brown-which is just white with 'caramel' colouring), drink well (water) and rest well, you may need more if you've just stepped up your exercise.
