What Causes Chronic Pain Below My Sternum?

In between My rib cage just below my sternum has been hurting for 2 weeks. The pain is quite obvious most of the time but then I also have episodes where it is unbareable and can't move and I also struggle to breathe. I've been to numeous gp's but they can't seem to find the cause and keep giving me antibiotics and drugs that don't seem to do anything. What can be the cause of this?