Stress, smoking, little or not enough exercise, wrong diet, insulin are all causes of those evil cellulite. You do not need to be overweight to have cellulite, you can even be underweight and have them. Getting cellulite while being pregnant is common because of among other things; hormone changes. I got cellulite when I got pregnant even though I ate healthy and exercised. Cellulite are uneven fat that are stuck in lumps and are most common to happen to women; estrogen is said to play a big roll.
You have different types of cellulite depending on where they are placed on the body and the hardest ones to get rid of are the ones located where the blood flow is the worst, like back of the upper leg, right below the butt.
How to get rid of them:
Cardio exercise and weight lifting to help release the fat through more blood flow. Athlete massage (to improve blood circulation) and a good diet. Exercise and a low calorie diet will release the fat and make it softer so that it wont make lumps. And eventually the fat levels will be low enough to erase the lumps all together, it is going to take a while though so do not give up.
Put lotion on your body every day to keep the skin elastic and do not eat heard fat. Hard fat stores easier and ends up as cellulite.