Hi Vicky, oddly I've just posted something about heart arrhythmia in the training forum.
A few years back I had an arrhythmia which I think was brought about by drinking a ton of coffee one day then havging a stressful morning the next day and going for a run at lunchtime.
If you have an occasional arrythmia it might be detectable in your pulse but will definitely be detectable my an ECG which your GP can sort out. I would err on the side of caution and go and see your GP - preferably when you're having the pain.
Is there any chance it's trapped wind? That can appear in the chest and longer distances can shake the gut up a bit. What and when are you eating and drinking before your runs?