Answer:
During a workout your body's blood flow increases which would make your blood vessels dilate. This will often make your veins stick and and making you feel pumped.
After the workout as you begin to rest the blood vessels will return to normal size and your heart rate will be at a normal rate and this will take the pumped feeling away.
Also your body does not loose the muscle you must remember your body grows during recovery not in the gym.
Hope this helps
TRAIN HARD REST WELL :)