I will try a guess at the reason for your excess energy: Because you were unfit, starting the exercise program caused an increased demand that exceeded the existing capacity of your body, and your body is now trying to modify itself to be able to respond to that increased demand. As a result, you have a higher heart rate, and in general a higher metabolic rate.
How do I know? The same happened to me whenever I increased the intensity of my cardio workout. But it won't last like that forever: Your body will eventually catch up with the demand, and then you will become fitter but not euphoric with too much energy.