I think you need a CAT scan to rule out Pulmonary Fibrosis, a CAT scan will show a much better picture of than any Xray. Is your doctor a Pulmonologist? If not, make an appointment with one. Try to find one who is a Specialist with PF, a University Hospital is a good place to start. I had a regular Pulmo first and I lost more lung capacity by scarring being treated by him. Perhaps it is the anxiety, but lung function is nothing to fool around with. So, I say check it out.
Good Luck.
I think you should go with what your dr. says. If he had seen anything serious he would have told you and started treatment. If you don't trust him, then you should find another dr. Unless you have a medical degree yourself, you have to accept what they tell you. And don't worry so much. Worry can cause physical symptoms on its own.