I mean, if you have a pump and then drink that big shake, your pump is going to go away because a lot of that blood will start being drawn to the digestive system to aid in digestion.
Most likely the decrease in arm size is a result of a decrease in glycogen stores. In a week's time, my arms may flucuate in size around 1/2 inch. Some days my arms look large and full, other days they look softer. Has a LOT to do with glycogen, so don't worry that you've lost muscle mass. Usually you don't lose a noticeable amount of muscle mass as quickly as you would notice a decrease in volume as a result of a decrease in glycogen... Yeah!
Anyway, maybe a nice little break is in order to recupe.
Also, I'd recommend waiting to drink such a big shake. After I workout I like to take some sugar (dextrose/maltodextrin or waxy maize) and then some whey protein. Then about 45-60 minutes later I'll eat a larger meal.
Good luck man