It's another example of Lynn stealing from Guisewite.
Panel 1: We start with her at the fitting rooms in a clothing store; she has Lizzie in one hand and a black bathing suit in the other.
Panels 2 and 3: She smiles because she looks pretty good in the suit.
Panel 4: She then stops sucking in her gut and thought-bubbles about how she can't wear it for long without passing out.
Summary: I think I know why Elly was pissed at John yesterday; he probably said something evil, cruel and unfaaaaaaaaaaaaaaaaaaair about how she was responsible for how fat she was getting. It's not her fault that she's losing her looks, it isn't. It's the fault of everyone else in the world and if people don't realize that, they're mean and evil. She has no patience for the phrase "making a mountain out of a molehill" either. She's hideous, do you hear, hideous. John is obviously lying when he says that she's not fat and ugly.