What is going on with these lines on my cheeks under my eyes?

I guess after the stomach flu, you may lose some weight, and those lines became more noticeable, but most probably you had them even before the flu. You can benefit from a filler like Belotero in the U.S. or Juvederm Volbella, that is not approved yet in the U.S. but is approved in Mexico and most of Europe. Volbella and Belotero can be injected more superficially, with less chances of getting a Tyndall effect on the injected area. Volbella in particular can last for up to 12 months, very minimal discomfort with the injections, as it has lidocaine .3% and minimal recovery time.

Facial volume loss do to ageing is a slow, gradual process.  Perhaps dehydration has accentuated these creases or have made you more aware of the creases.  Wait several weeks (6) to see if there is improvement. If there is no improvement a hyaluronic acid filler to the tear trough and cheek creases will improve the appearance.