Is this GERD or angina?

At your age, it is likely to be GERD which can cause intense chest pain which can be confused with heart problems. I have had this intense pain from GERD and the first time I had it, it frightened me because I didn't know the cause. I took painkillers, it gradually subsided. You could try milk which helps soothe the stomach along with your antacid. Gas build up can cause this pain too. If you are stressing about it, it will make the GERD worse and cause it to be continuous. Do not consider angina; it will only make you worry more, Instead, see your doctor, describe your symptoms and worries. Ask for a stronger antacid such as a PPI or a higher dose of the one you are taking. You will get reassurance from your doctor.

Do you take any medication for pain like ibuprofen or aspirin? These can cause inflammation and discomfort in your oesophagus and stomach.

It could be costochondritis. Does the pain get worse when you press it?

I would think angina would be highly unlikely at your age and from the description you give it really sounds nothing like it. I have people in my family with angina and they wouldn't describe the pain mild and dull and just in the center of the chest. It is a very unpleasant feeling and when people have an attack, they look terrible and usually have breathing problems often sweating and going very pale.