Thanks for you response. I undertook spirometry and lung capacity tests yesterday and all of my results were normal (better than average) so the doctor has ruled out athsma. I now have a referral to a pulmonologist for next month. It seems like a long wait, like some people are prone to doing, I am thinking the worst and have convinced myself I have some sort of fatal illness, so I will be counting down the days until I see the specialist.
If the repeat chest x-ray is truly clear, that would effectively rule out the diagnosis of a tumor, benign or malignant. It would not, however, rule out the possibility of localized bronchiectasis, often seen with cystic fibrosis (CF), or the presence of a non radio-opaque, meaning not seen on a plain x-ray, aspirated foreign body, such as food, that could be the cause of obstruction and chronic inflammation. Since your problem seems to be on 1 side only, this argues strongly against the diagnosis of asthma and allergies being an issue. However, if your doctor continues to seriously consider the diagnosis of asthma that should not be ignored. Pulmonary function tests (PFTs), before and after the administration of a bronchodilator and a methacholine challenge would help settle the issue.
The most important concern is the persistence of respiratory symptoms, including cough and phlegm for 5 months in a previously health 29 year old, which remains unexplained. This must be resolved. A CT scan of your lungs would ordinarily be the next step, in this circumstance, often followed by a bronchoscopy. CF remains a consideration and testing for it readily accessible.
You should ask your doctor if, given the persistence of your problem for 5 months, consultation with a lung specialist, also called a pulmonologist, would be appropriate. One way or another, the cause of your symptoms should be identified and treated.
Good luck.