What Causes Pain In Shoulder Blade?

I'm 22 years old, 5'8", and 170 lbs. I recently had a child a month ago but I've had this burning/ stinging sensation in the bottom of my left should blade for about three weeks. The sight of the pain has also been pretty swollen. It started when I was holding my daughter and progressed to the point where it is practically impossible to be without pain if I use either arm. Recently is has become worse and over the counter medications have ceased to dull the pain as well as putting ice or heat on the location. When I do get pain I have to stop the task at hand and squeeze my shoulder blades together otherwise the pain intensifies till I'm near tears. I'm seeing the doctor next Friday but would like to at least get an idea as to what it might be.Krystal