A mistake a lot of people make is that they assume that the human body is some kind of book keeper. That it counts the calories in and calories out and whats left is the energy you have. Unfortunately its not that easy. There are hundreds and hundreds of other variables that have influence on your energy or health in general.
The biggest calorie burners are your digestion and your internal heating system to keep your body on the right temperature. But it's not about keeping track of the calories. Getting energy is about providing your body with the right nutrients (including water and oxygen) and supporting your body with eliminating the toxins.
If we take a look at our bodies a bit closer you see that your body is build up from cells, those cells provide us with the energy we need (this process is called metabolism). Providing those cells with the chemicals they need and eliminating it's waist products will result in an increase in energy.
Above all cells need oxygen and water, so take deep breaths and eat a lot of foods that contain water (vegetables and fruits). Besides that cells need vitamins and minerals, so make sure you eat a lot of vegetables and take some high quality supplements). And lastly cells need sugars (carbohydrates are converted into sugar) and 'good' fats.
To answer your question, it is very important to listen to your body. If you feel tired you need to make some changes. There are a couple of things you can do. I listed the important ones below, starting with the one with the highest impact:
Study yoga breathing techniques and learn to take deeper abdominal breaths
Eat more water rich foods (at least 60% of your diet should be vegetables)
Sleep more (ideally you want to wake up without an alarm clock most of the time)
Exercise less or less intensive (walking is considered the healthiest form of exercise by many doctors)
Take vitamin supplements and eat more omega-3 fats.