Because she said, "You're giving yourself gas!" Okay, now she meant fuel but in American English,  

gas, gasoline also means the pressure you  feel inside bloating which ultimately leads to  

farts. Now, in other English speaking  countries, they don't use gas. They use  

petrol or fuel for example. But in America,  gas is what you put in your car. It is fuel.  

But when we talk about gas in a person,  if something gives you gas for example,  

"Broccoli gives me gas." That means it,  you know, it messes up your digestion.  

So she said, "You're giving yourself gas." We all  knew what she meant but it sounded pretty funny.