What Causes Severe Spasms In My Thoracic Diaphragm Area?

Hi. I am a 45 year old female, height 5'5" and weighing 10st 7lbs. I took up a fairly vigorous exercise regime when I gave up smoking 6 months ago. For around the past couple of months I am frequently awoken at night with increasingly severe spasms in my thoracic diaphragm area. I have had an ECG which gave a normal result. Full blood tests have not revelealed anything either. My GP has put it down to anxiety but I don't think this is correct. Can you suggest any possible alternative diagnosis?