Entering puberty causes many changes to the body. The energy it takes to make these changes can certainly sap energy. You will probably need to change your diet, an increase in calories is not unreasonable at this time. A balanced diet and enough sleep should go a long way towards mitigating the effects. A multivitamin may help as well. Consult your physician next time you are in and if it is affecting your ability to function normally, make an appointment!