The Fairy will only ask you for 10 watermelons if you have taken her home with you. To take her home with you you need to feed her different fruits. It goes from strawberry up to watermelon if i remember it right. With every visit to the Fairy you can increase the fruit size by 1, if you try more she will refuse to eat it.
So what happened here is that the game is trying to get you to encounter the scene where Beth gives you her phone number , but in this current version that scene is only available at stage 4 and 5, so if you gained weight past that point without getting the number it's just going to keep making you encounter Beth, its a know bug. I was going to wait till the next version to patch it out, but I think enough people have encountered this problem that I will put out a small patch to address it.