Stitch/Cramp in Solar Plexus

Can anyone help me?

I have always been a sufferer of stitch when I run, but can usually run through it - although it is annoying because I know I could go faster if I didn't get it. Lately, however, when I do try and run through it, the pain moves to my solar plexus and is excrutiating, meaning I have to pull up. Then it is as though my diaphragm has cramped and I can't get breaths into my lungs. I then panic and make it worse. I have tried using Ventolin at this point, but it makes no difference. I have been to see my GP today who is at a loss to explain it and is sending me for some tests. However, I wondered if anyone else has suffered from this. I see alot of threads about side stitch, but this is right in the centre. I do not get it when I mountain bike, swim sprint, I teach studio cycling and aerobics and don't get it then, rowing - anything else aerobic. I only get it when I run and particularly if I increase the pace. 

Any advice?