Can too much gas build up with a hiatal hernia feel like you can't breathe?

Yes: Yes, swallowing a lot of air or consuming air containing beverages, for example carbonated sodas or water can fill up the stomach including the portion of stomach above the diaphragm when there is a hiatal hernia. This can result in pressure or fullness senastion in the chest and even shortness of breath. Excess swallowing of air aka as aerophagia can result from chewing gum, drinking a straw.

Possible: It is possible, but generally only with large hiatal hernias, also known as paraesophageal hernias. I have never seen one of these large hernias in someone in their 20's. It is possible to have different types of diaphragm hernias that could cause those symptoms. These are called bochdalek and morgagni hernias. Hope this helps!