Physically dragging?

Oven been working out for just over 3 weeks, and today has been a "rest" day for me.... My question is, the days that I work out, I feel like I can just keep going and going and have a lot of energy all day, clear up to when I go to bed....but on the days that are "rest" days I just feel draggy and don't have much energy...my diet has not changed any, so I can blame that...so does anyone else go through this? And... What do ybdo to overcome this feeling....?

This has been happening since week 2 of workout....so this is not a new thing...