Tired and Weak? Should I be worried?

I am an 18 year old girl who is very active during the day and usually I have plenty of energy to get myself through a hectic week. Within the last four months my energy levels have diminished. Not only that but I have been sleeping more and eating more just to get enough energy to go outside and be active. I have not gained any weight but the only way I can get any energy is by eating and even that does not work sometimes. My eating habits I might add are good. My family is a health "freak" my dad suffered heart problems so we have to watch what we eat anyway, but I honestly do not know what to do. My doctor said I was fine, I was tested about a year ago for mono but even then my energy levels were higher then they are now? Anyone have any suggestions!? I get a good ten hours of sleep a night, sometimes even more, I drink plenty of water, I eat all the protein foods I can find... I am stuck!