Hello, Process of acute bronchitis preceded by either viral or bacterial infection leading to inflammation of bronchi causing formation of a lot of mucus. Due to infection you have got fever but this may occur due to complications like pneumonia. coughing a lot to expel out sputum causes upper back pain. So you need to consult a good physician for further evaluation. Before that go for blood examination- CBC, chest x-ray, sputum culture, lung function test. Continue your medication and take rest with good amount of water. take care, bye.