Can't seem to lose my access weight.?

Okay I am only 18 years old, and always been considered an extremely tiny, underweight. Everything above my waist is skin and bones basically. I have noodle arms, bony wrists, flat stomach (small boobs :/) and just petite. But my butt and thighs are HUGE! I know my diet isn't the best, cause I do enjoy my cheeseburgers and french fries but I rarely eat them, and my parents (who are always both a little over weight) are very good at making healthy foods, making sure we all get the right diet we need, and have each food category in us, like wheat, whole grain, dairy, protein, iron etc. But I seriously cannot lose this bottom weight! I walk everywhere, and even bike and yet it seems my butt and hips get bigger and bigger. Especially my butt. In jeans it doesn't look it but in shorts you can tell I've gained some weight, and it's embarrassing because I have small stretch marks from the sudden weight gain and some cellulite, though no one really sees it. I can feel my legs and butt jiggling when I move and I feel bad for whining about it, because I know there are much heavier people out there, some who can't help it, but seriously...I feel like it's just gonna spread up to my upper regions too! I've always been fit and small with a good body. But my bottom half is just so disproportional to the rest of my body! lol. It makes no sense! How do I lose this weight!