That weakness feeling happened to me some months ago.
After workout, or even when you wake up in the morning you don't want to do anything, you feel shaky.
For my experience I can tell you: eat better (sometimes I should say "more") and don't exceed with workout.
After some weeks of workout you think you can do more and more, and yes, you can do more but after some days you will feel as shit. Then is when lot of people leaves the gym and starts again a more sedentary life. In that moment starts the rebound effect (will talk about it), and all your previous efforts were for nothing.
When you have that feeling the best you can do is still going to the gym, change the workout if needed to another one more soft, and eat some more/better, because one of the most popular thoughts is "eat less, training hard, get thin faster", and that's not true.