From what I've read, I think that is your central nervous system being amped up from the workout. At first, fatigue masks the "amped up" feeling, but as you recover from your workout, the feeling of being amped up shows through for lack of a better description.
Fairly normal, for that reason, I don't workout too late at night as it causes me to have trouble falling asleep because I feel like I can't shut off my brain.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano