Have you heard of something called Costochondritis it is:
is an inflammation of the junctions where the upper ribs join with the cartilage that holds them to the breastbone or sternum.
It generally goes away on its own and is not a serious thing at all. Can be caused from a hit to the chest, i believe.
Maybe its something to look into if you already havent!
take care

I have looked into it but I didn't know if I needed to see a doctor about it. Do you know if they can do anything to cure it? It said on the internet you could take NSAIDS for it but I can't take those because I have an ulcer in my large intestine.

Check the websites with people who have ulcers,reflux etc. The chest pain might be related. I have gastritis and ibs and when I have flare-ups, my chest hurts and pops when I stretch. It doesn't in between flare-ups. Hope I helped. KC

I have a sore stenum and wish it would pop more often. The pressure and pain (just to touch the sternum) is very uncomfortable. I can say that a pain sternum and acid reflux are not the same. I had acid reflux very bad until I started taking Nexium. The acid reflux has gone away but the sternum problem has persisted. I have had problems with my sternum popping and pain for at least 20 years. It has gotten worse in the past couple of years. I have been told I have arthritis in my sternum, since it is basically a joint of sorts. I have contributed the pain to being a little over weight and the sternum is pushed out due to the outward condition of my stomach. That and the fact that when I sit down the condition is worsened. Taking off weight and sitting up straight would be some helpful tips. Taking aspirin or something that relieves imflamation has been very helpful for me.