Why am I so tired?

I know I'm not pregnant cos I haven't had sex before but I go to bed before 10pm and I wake up at 7 but I'm always so tried..I eat breakfast lunch and dinner with snacks inbetween and I usually do nothing during the day and the only excersize I do is take my dog to the park and do 30 sit ups and a plank for 5 minutes every 3 days, I just don't get why I'm so tired, if anyone knows what could be the problem please comment..willing to do anything to stop the tiredness