I've just started smoking again after quitting for 3 weeks (not just weed, but cigarettes, alcohol and all other drugs too). When I first stopped I felt what you've described - it felt kinda like a withdrawal symptom. At first I thought that I must have been addicted to weed but after the first few days I felt fine again. I think that it wasn't withdrawal symptoms, but just me getting used to the idea of not doing all that stuff - getting used to the change of habits.

And yes I did go to gym, but not anymore than usual. I wasn't anymore active or inactive while I quit, and except for those first few days I was fine.