The fact that you're losing "weight" but getting "looser" should SCREAM a giant clue directly into your eardrums........losing "weight" does not make you healthier, nor is a good result to shoot for in and of itself.
You're losing muscle mass, but likely increasing your fat stores. Even if you're not increasing your fat stores, I'd still bet money that you're slowing down your metabolic rate. Then, in the future, you're gonna have an even harder time keeping the fat off, keeping the muscle on, and it'll be extremely difficult to get your youthly butt-shape back.
But if you want exercises to work the butt, search for "glute exercises" in Google and pick something from a reputable source.
Aside from that, if you have normal testosterone and estrogen levels for the average adult female, and unless your legs are predisposed to becoming bulky, I seriously doubt that you'll need to worry about bulking up your legs to any significant degree, even if you do target them for weight training.
p.s. I don't think that "looser" is a word
