How can I have more energy in the gym?

[Curious College Student]

Is there any way that I can have more energy when I'm weightlifting? Today when I went to the gym I had little to no energy when I was working out. I eat plenty during the day, drink about 5 bottles of water a day, and I get a good 8 to 9 hours of sleep every night. So why is it that after about 15 minutes of weightlifting in the gym I'm tired and burned out? I mean, yeah I get a little bit "pumped" when I'm done leaving the gym but it goes away by the next day and I feel that nothing was really accomplished..

How can I get more energy, and look more pumped after leaving the gym?