Mamalace,
Hello and welcome. This is Kitt and I am so happy to meet you.
It may be diaphragm spasms that you are experiencing. There is also a tic-like clonic spasms of abdominal and diaphragmatic musculature that can be either unilateral or bilateral.
Medications can sometimes cause this phenonmenon or as you have mentioned the carbonated beverage such as Diet Pepsi.
Please remember I am not a professional and if this continues you need to review the problem again with your physician.
Perhaps other members have some experience that will help you.
Again Welcome and stick with us.
Kitt