On this incredibly hot and tense afternoon at the Buchanans' house, Daisy eventually suggests that she, Tom, Gatsby, Nick, and Jordan all go to New York City. When Tom casually remarks that "you can buy anything at a drugstore nowadays," he seems to be getting in a dig at Gatsby, who he now knows to be responsible for the illegal sale of grain alcohol at drugstores across the country. This is the era of Prohibition, and so Gatsby's activities make him a criminal.
When Tom stops the car he's driving—Gatsby's car—at George Wilson's gas station to fill up the tank, George's wife, Myrtle, with whom Tom has been having an affair, looks out the upper-story window and sees Jordan sitting in the car with Tom and Nick (both of whom she knows). She's never, however, seen Daisy, Tom's wife before. Nick says of Myrtle,
[. . .] I realized that her eyes, wide with jealous terror, were fixed not on Tom, but on Jordan Baker, whom she took to be his wife.
Thus, as Nick says, Myrtle is upset when she sees Jordan because she thinks that Jordan is actually Daisy, Tom's wife. Myrtle becomes incredibly jealous because Daisy occupies the position in Tom's life that Myrtle would like to have. Remember, earlier in the book, Tom slaps Myrtle for saying Daisy's name, breaking her nose. Myrtle wants to be the upper-class woman that Daisy is—she wants Daisy's status, money, and husband—so when she sees Jordan, thinking Jordan is Daisy, she grows livid with jealousy.