I'd recommend you go to a doctor
I feel constant pain and pressure on the left side of my rib cage (it feels like it's coming from underneath my ribs) about middle ways down from my sturnum and pain in the middle of back between my shoulder blades. What could be causing this pain?
Well, sounds like what I get when I run too much. If it dies down I wouldn't worry about it but you should definitely consult a doctor, or Mary Poppins.