I'm 18, woke up with chest pain, hard to breathe. lower throat feels swollen, post nasal drip that hasn't stopped for years. coughing. cancer ??

Not likely.: at your age and with your symptoms cancer would not be the likely diagnosis. But, with such long lasting symptoms in the background and new symptoms now you should consult a care provider for help.

Chest pain: There are several possible causes of your chest pain, but first would talk with your doctor and try and treat the post nasal drip. Other possibilities are gastroesophageal reflux or even a virus. At this time your symptoms are not representative of any malignancy