I constantly feel tired no matter how long I sleep?

I go to bed at 12 and wake up at 7:30 to babysit for 10 hours. Everytime I wake up though I feel dead and it's impossible for me to get out of bed. I take a nap when I can for an hour or two but I'm still tired. I used to have energy to work out for 3 hours now I can't even go to the gym for 15 minutes. I recently went on more depression medicine but it's suppose to keep me awake. What could be wrong with me?