Luna has thrown up from not Heartgard but Drontal Plus. Did she get it on an empty stomach?
It's so hard because they can't tell us what's wrong so we automatically freak out and think it's a 911 emergency. Sometimes it is, sometimes they're just feeling blah.
If it were Luna, I would wait-and-see for 24 hours and then reevaluate. Sending healing thoughts.