Bingo. It's a plumbing issue. You're dehydrated throughout the body, but when you dump some fluid into your gut, much of your freshly rehydrated blood will head on to the kidneys and lose significant water before that water can be used to bring you back to full hydration. It's going to take a while to rehydrate, but you can minimize the time by:
1. Don't get as dry (duh). Drink more when you ride and before, even if you need to then take a natural break now and again.
2. Drink many smaller doses over several hours rather than one big hit.
3. Keep moving. part of re-equilibrating blood and other tissues is mixing the fluids. Contraction of muscles helps do this, so go for a walk in the evening after you ride.