Or rather, she would have. But she started feeling severe pains in her legs, and immediately sat down.
"Ow... what... what the hell..." she mutters, rubbing her thighs as the changes progress ever so slowly. Now that she wasn't standing on her legs, the pain was subsiding.
"No way..." she whispers, putting two and two together. There's a reason why cheetahs are so lithe and trim - their legs can't handle the amount of weight her body's putting on them. She wouldn't be able to work off the weight as a cheetah if her legs broke under her own weight.
She frantically reaches for the box once more, downing the human vial until yet again, she's back to being an overweight human being.
"Okay, so scratch that plan..." she says as she contemplates what she should pick instead.