A few years ago I was playing icehockey maybe once a day or something and I felt great, school was easy and I had a lot of energy to spare (well, for a teenager anway).
Then I quit hockey and I started doing nothing, just sitting in front of the computer and guess what happened? I got inexplicitly tired and burned out. I love math but it got boring and my arms where like spaghetti and I almost tanked school (thankfully I didn't). I've always thought it was lack of challenges but now I know it wasn't that simple.
I think that my lack of working out really did make me more tired, and not just a bit but extremely tired and depressed and a whole heap of things. Now when I've started at the university I had this same strange sensation like back then, much milder but still familiar. So I went to a couple of taekwondo practices and I'm feeling a ton better already.
So my suggestion is to start a regular, and then I do mean regular with no skipping because you don't feel for it or don't have time etc, workout. You don't have to go for a run if you don't think that's the most fun thing there is (I personally really dislike it) but find something fun - ball sports, climbing, dancing whatever.
Definitely worth a try!