What is that belly called in English which is bulging out/swollen?

A paunch.  

https://www.merriam-webster.com/dictionary/paunch  

"Potbelly" is also widely used as is "gut" which really means intestines but we say, "his gut was hanging over his belt," or "she'd developed a bit of a gut after all the party food at Christmas."  Not to be confused with "guts" in the plural for bravery/courage.

For men it's often called a "beer belly or a "beer gut."

Beware that all of these terms can be considered offensive in English.  People don't like drawing attention to a big belly (unless it's a pregnant woman).  Many people consider an overhanging belly to be a moral failing.  In past times when lots of food was expensive growing a large belly i middle age was considered to be a sign of success, so if you ever read Victorian literature from the 1800s their attitudes were different for the modern one.

Pot belly, beer gut, fat.. take your pick!

Are you thinking of "distended"?

Or, run-of-the-mill "beer belly"?