I am going to move this out to the main forum, as per forum guidelines, and also because that forum moves more quickly. What you're describing is a frustrating situation, but it's also fairly common, and I think you'll find that out on the main forum, you will get more support.
I've dealt with slow letdown before. And it's frustrating. It's especially frustrating because it seems like the more you worry about it, the worse it gets, because the stress can inhibit the letdown reflex. Pumping mamas go through this, too-- when they begin to think they're not pumping enough, the added worry inhibits how much they're able to let down for the pump, which in turn makes the problem worse.
It's important to remember that letdown is a reflex with a strongly psychological component to it. It can be conditioned-- trained, if you will. I trained mine once, accidentally, to respond to one page of my Dr. Sears Baby Book, the one where he describes the process of letdown. I would find myself having trouble lettting down, and in frustration I'd reach for the book. Then once, just as I was reading that page, the letdown happened to occur. Soon, every time it was slow, I'd open to that page, start reading, and Whammo! there would be the milk.
My point is that routines and little rituals can really help. If you start to make a habit, whenever you can, of doing things in the same order each time-- sit in the same place, say the same words to baby, drink a glass of water before you begin, put on the same music-- having those little habits can start to condition the reflex to happen more easily. Also, does your baby fuss and thrash around when the letdown is slow? I've come to believe that the thrashing and fussing is the way that babies encourage a faster letdown. Switch nursing can also help-- that's when you switch sides multiple times during a single feed. It's a classic supply-increasing technique.
I always did find that my supply dipped during my period, once my period had returned. My DS used to take two hours to fall asleep, the first few days of my period. But it always seemed to rebound after a few days, especially if I kept on letting him nurse frequently.
I think it's extremely unlikely that you're losing your milk. It doesn't just dry up with no explanation.
I wouldn't go overboard with the hydration. Enough is great for supply, but too much can have the opposite effect. Drink to satisfy your thirst, but there's no reason to go crazy with it.
I really wish I could be of more help. I do wish you the best, though. I know it's easy for me to say relax, since it's not me that's struggling- but really, relaxing may be the answer, so if you can find some way to take your mind off it while you're nursing, that may help.
Best of luck!
Follow Mothering