I Love To Pretend To Be Extremely Pregnant

So this is very early, but I want to tell my grandma I pretend I'm pregnant so that next time it's just her and I for a few days I can keep my belly on around her. The one thing keeping me back from telling her if that I don't know how to bring it up. Now that's where my supporters here come in, I'd like to know how I should bring it up, and if it's a good idea.
(I know everyone has different opinions on these kinds of things but I'd like to hear advice instead of the obvious)