Why do I vomit when I start eating after being hungry?

I'm 19, 5'6", 105lbs., and female. I smoke cigarettes. I also drink alcohol and smoke marijuana on occasion. I have a very small appetite so I don't eat as often as I should. I mostly drink Dr. Pepper but half of my diet consists of fast food and half hamburger helper type meals at home. Sometimes, when I'm hungry, when I start eating I get nauseous and after a few bites I start vomiting. Also sometimes when I first wake up and smoke a cigarette after the first drag I get nauseous and if I continue to smoke I eventually start vomiting. Sometimes the smell of food cooking and smoke makes me nauseous. This has been going on almost my whole life so I know its not pregnancy. I have a pretty violent cough and always have, sometimes if I cough a lot I start coughing up a lot of phlegm and sometimes I vomit. I've lost about 15lbs. in the last year I don't know if that's connected. I don't feel like this every time. It comes and goes at a random rate. What's wrong with me?