yah, not good to lose so much weight so fast besides the fact that it can make your skin sag especially around the chin and under the triceps,
if you lost it cuz you had the flu or something that give it a few weeks and your body should return to normal. If you lost it due to crash dieting or drug use that you plan on continuing than that's what you get for forcing your body into unhealthy weight loss... weight lost in the wrong places. You can't program your body to gain or lose in one particular area. However, to gain your booty back eat normally and work out on an eliptical or stair master plus leg presses, that should do the trick.
my advice only, but I've been working out with a personal trainer and it's the kind of advice she'd give me