Yeah, it probably is a combo of glycogen depletion plus water loss through sweat... Although I've gone through 2+ litres of water during workouts before and still get the feeling... my workouts tend to last about 90 mins and seems to occur at about the hour mark then feel flat for several hours after till I get a few meals in me. Ive tried every kind of nutritional combo around my workouts high cals, high carbs, high protein, bcaas... never really made a difference.... kinda learnd to just live with it now, just need to minimize all human interactions during those hours 
Well there's your problem..
No SHIT you're gonna feel like shit + small, flat, deflated and weak post workout after training for 90 mins.
Why would you workout for longer than 45 minutes??? 10-14 sets and you're DONE. WTF do you (& others) do for 90 damn minutes? Sit around for 10 mins in between sets checking out ass? Do your set, which takes what.. 30 seconds? Rest for 2 mins, repeat x12. That's about 30 minutes.
Drink about half a litre right before your workout, about 150ml between every set (don't worry what the fat insecure ignorant homos say about constantly drinking water and carrying a bottle around the gym), keep your heart rate up by taking shorter rest periods + flexing/squeezing your muscles between sets.. and you'll walk out of there 40 mins later with an epic pump feeling god-like.
Note for noobs & idiots: The pump = fluid intake + workout intensity (& obviously dbol, test... for the juicers). If you don't drink enough, you'll NEVER get fully pumped. If you don't train hard/fast enough, you'll NEVER get fully pumped. After around the 50 minute mark, for whatever scientific/nerd-like reason, the excess fluid/blood starts leaving your muscles (unless of course you're on the sauce) deflating you. Naturals who say "I stay pumped all day after working out" are delusional.
"Approved" Approved!
