Why after drinking a diet soda do i get chest pain and quickly burp up a lot of white foam a couple of times before i get back to normal ?

The fizz in a diet or any soda is from (CO2) & carbonic acid which releases gas. It will continue to release gas adding to the air bubble normally seen in the stomach. Burping afterwards releases some of the stomach bubble/pressure. The stomach entrance is just behind & to the left of your heart & the action/acid likely irritates that location. The lay term is heartburn.

Ir sounds like the carbonation is not agreeing with you. Perhaps you should avoid carbonated beverages for a time and see that it gets better. You can also see a GI doctor to find out if there is anything wrong physically or if this is a sensitivity. You could also see an allergist to see if there is an ingredient you are allergic to. Good luck!