I have had pain in my left shoulder blade for several weeks - worse when i am carrying something? also some back pain on left side.

Multiple causes: Upper back & left sided chest may be the result of a thoracic disc herniation, thoracic facet syndrome, T4 syndrome, costoveterbral syndrome, muscle strain, ect. It may also represent referred pain from internal organs such as the heart & lungs, which are more serious. With your breathing difficulties you should contact your pcp for an immediate evaluation. If not available go to nearest er!