Since I woke up this morning I have had a pain in my chest - right in the middle (and no where else). It feels like muscle/bone pain - as if someone punched me in the chest. But I have no bruise, no chest infection (ie no coughing, but I have been sneezing but thats probably hay fever related) and I didnt do any strenuous exercise yesterday. Anyone have any ideas?!

you may have "slept wrong". on your side, with your shoulders pushed forward.  don't underestimate sneezing.  I've seen patients with broken ribs and often displaced disk in their backs from sneezing too hard or stifling a sneeze