Chest tightness, cough, mucus tastes of blood, tired. How do doctors know it's not pneumonia?

I am a 25 year old female. Around four weeks ago I became quite ill with a severe cough that became that bad it caused me to vomit and have to sleep sat up in bed. Doctor listened to my chest and thought I had an infection. I was prescribed co-amoxiclav 625mg for a week which did not work. He then prescribed me clarithromyicin 1000mg per day for a week. This again did not help. The tightness in my chest however began to get much more severe one evening and I was rushed into hospital unwell. Doctors gave me a nebuliser which immediately helped me breathe better. I have been given an inhaler to use regularly. However I do not feel that this is asthma . How do they know it s not pneumonia? I have a tight, sore chest with a recurrent cough that is worse in morning, evening and with the cold weather or exercise. I am coughing phlegm up but not enough so that I can spit it out and see what colour it is. The mucous tastes of blood also. I feel persistently tired and the cough has now began to wake me from sleep. Could someone please help? Also had a chest x ray to rule out a pe and this was normal? Would pneumonia show on an x ray? My chest is wheezy and i feel like a bubbling sensation when I breathe.