Why am I always cold?

Hi, I'm a 15 year old girl. My height is 5'8" and I weigh 138lbs. I'm always cold, especially in school. I'm even cold in my warm uniform sweater. I don't eat much but I'm not anorexic, I'm just not hungry. My daily diet consists of an omelet for breakfast, tuna sandwich for lunch, and salad for dinner. I rarely eat meat. If I do, it only happens on holidays. I barely exercise as well. Do you think this is causing me to be cold? Any advice is appreciated. Have a nice day!