Hi
Welcome to the MedHelp forum!
A chest pain some times on the right and then on the left can be due to acidity and gastric reflux. However since you also have difficulty breathing, phlegm, it would be wise to get a chest X-ray to rule out pneumonia. It is very important to consult a doctor immediately to rule out pulmonary embolism as cause of chest pain and breathlessness. This is a medical emergency and since I cannot diagnose you on net, based on your symptoms I would be comfortable if this is ruled out first.
Hope this helps. Take care!
Yes it could be your thyroid,it could be getting larger or you could have a growth on it,please see your Dr asap to have the proper tests done,your doc may send you to an ENT dr.(ears.nose and throat),but I would not put this off.Listen to your body,it speaks the truth!