Have you been counting calories carefully, to see what you're really eating? It's possible you're eating more than you think. And powerbars and gatorade do have calories...

Also, have you been weighing at the same time every day? Best time is early in the morning, without clothes, after you've *ahem* relieved yourself, and before eating or drinking.

FWIW, My weight "loss" pattern is up and down all over the place, from week to week. Last week I "gained" 6 lbs, this week I "lost" 7, but the general trend is down. Three weeks may be long enough to give a general trend, though...

I have no idea if 8 lbs of muscle is a reasonable gain, but if you'd lost fat, but gained 8 lbs, that would be more than 8 lbs of muscle.