Poor diet? It's hard to say without knowing if your diet is well balanced. If you're not getting enough protein you may feel tired. Exercise usually makes a person feel more energetic. If your lack of energy continues see your doctor. You might want to keep a food diary for 3 days so your doctor can see what you're eating.
Or hormone imbalance. Lack of energy is one of the main symptoms of hormone imbalance. However, it can be deconditioning - related to lack of exercise or improper nutrition.