Here there could be two resion for your pain First it can be due to muscular weakness or soreness in muscle near to shoulder blade and so that the normal biomechanics is also affected for shoulder joint. And so that it pains when ever you move it. Second cause could be the chest infection. Here you have told that all the time you have cough there is sputum coming out and you smoke regularly. There is one more possibility of combination of both above issue. Here what so ever it is but you need to consult a doctor need to take chest x ray and shoulder blade x ray to see what s wrong with it in side. And based on that plan the treatment with antibiotics, muscle relaxants or pain killers, as per doctor suggestion. Here if it gets diagnose as muscular pain then do visit physiotherapist and take physiotherapy treatment with postural as well exercise suggestion. This will give you a good long term relief. You can also use hot pack at home on your painful shoulder blade to have some more relief by relaxing effect on soft tissue. Take care.