Done x-ray and told my rungs are ok,though put on erythromycin tabs and no improvement.what other tests should i undergo to b sure of my lungs health?

Antibiotics: You may still have a lung infection even if your chest xray is negative. Take the antibiotic as prescribed by your doctor. Once you are feeling better have them assess your lung health with a spirometry test.

Give it time: The majority of coughs are due to viruses in the respiratory "tubes" (bronchi), called bronchitis. Bronchitis does not show any pneumonia on x-rays because the actual lung tissues are ok. Antibiotics, like Erythromycin do not work on viruses, only time does. If you get a fever over 101, cough up blood, get short of breath or feel worse instead of gradually better, see your doctor again.