I overate and my stomach is swollen like a balloon.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means swell, so the verb version is to swell or to expand.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Someone comes up to you and asks, "Hey, do you want your stomach to swell like that?" How dare they ask you about your stomach. You respond with, "F@#$ (ふく) you!" It's none of their business how much you want your stomach to expand.