Diet soda does that to me too...  especially on an empty stomach.  It is horrible.   The chest pain occurs when your stomach acid splashes up from your stomach and into your esophagus.  It can also come from spasms with in your esophagus itself.  Your esophagus is the tube that connects your mouth to your stomach and it lies in your middle chest area.  Many people confuse the pain that occurs with these issues with having a heart attack, it can be THAT intense.    That having been said, don't ignore chest pain.  Talk with your doctor to make sure it is coming from acid reflux and get it treated.  If you have this kind of pain all the time you may need a medication to reduce your stomach acid.  If you are already taking medication and still having problems you may need your dose adjusted, a different medication or you may need to address lifestyle issues.   I personally, try to put something easy on my stomach first thing in the morning.  After that I am usually ok to drink my cup of coffee (because this chick needs her caffiene, lol).