You're clearly back from vacation since you're calling from home.
Tell your manager that you're back from vacation and that you're calling from home. Tell him that you caught a nasty case of the flu while on vacation, that you are feeling absolutely miserable and that you are requesting several days to recover from the flu. As others including @StephanKolassa and @Formagella have commented, state that you are wiling to provide a doctor's note on request.
Flu is no respecter of timing. If you were not on vacation and you caught the flu between Monday evening when you leave work and Tuesday morning when you are supposed to report for work, you're still eligible for sick days. If you were back from vacation and you caught the flu five minutes before you are supposed to report for work, you are still eligible for sick days. Your justification - and mine and everyone else's - justification for taking sick days is that taking sick days is what we all do when we are sick and we're supposed to report for work, IRRESPECTIVE of how or when or where we first got sick or whether our being sick is our fault, somebody else's fault, nobody's fault or everybody's fault.
If your manager has any good sense, he knows better than let you show up for work, cough and sniffle all day and distribute multiple doses of your flu to everyone around you including him.
If he makes you show up, make sure that he gets full credit if your coworkers drop like flies.