18 year old, female, non-smoker, with persistent cough and chest infection for 4 weeks, and getting worse. could i have lung cancer?

Very Unlikely: more likely that you have bronchospasm from the infection. See your doctor for treatments aimed at decreasing inflammation and spasm such as inhalers, steroids etc.

Not likely : 18 year old smokers do not get lung cancer, no. But four weeks of persistent cough is concerning. Could be sinus infection, walking pneumonia, even pertussis. You need a good history and exam by your pcp! Best of luck!

Need follow-up.: Lung cancer is not diagnosed this way. You should visit a local physician where he/she can order at least a chest X-ray and some blood tests to start with. I assume what you meant by "chest infection" is more symptom of congestion. The symptoms you have described can be caused by a range of conditions from acid reflux to asthma.

You need: to be evaluated by your doctor to determine the cause of your symptoms. In your age group, lung cancer is rare, so it would be an unlikely diagnosis.

Unlikely at your age: But should see doctor for an evaluation to find the cause.