Walken, I had some of that, and I got more energy as time went on. I figured the winter was not helping, and I was not doing a lot of physical activity (I know myself to be lazy about that...working out has not been one of my personal vanity things).
If it is bugging you and you can manage it, you should see what a doctor thinks. And if you think you can do more exercise, give that a try and see what it is like. And just in case: don't let the lethargy feeling discourage you about your progress in sobriety. In other words, don't let yourself think "this is the thanks I get for doing the right thing" if that is how your mind would work. On a related topic, I experienced a return to insomnia and I felt a little upset about it, but I had it in me to see it as a better thing than the alcohol, and I am still off it (into my early 100s now in terms of days). Keep going, I hope it continues to get better for you.