I think it could just have been a second round of side effects
Extremely unlikely. The only delay/re-occurrence known for Moderna is 5-8 days. Not 21 days, and not lasting a mere 18 hrs.
That's also extremely unlikely. One has an anamnestic response from the vaccine probably hitting a peak for Abs at day 8-10. Then 10-12 days later another anamnestic response to infection. Causing Ab titers to go off the chart. The virus got stomped be it Covid or Flu.
More likely is someone completely vaccinated, being infected, and asymptomatic, as a carrier. The jury is still out on Covid asymptomatic vaccinated carriers.
Absolutely. And I had no desire to go running into the EMR in a panic on a Sat night. If it had persisted then maybe. Out of curiosity, and nothing else.