arrhythmia's many times are benign..professionals are trained to triage more serious types. The appointment given fits the situation. If any symptoms develop there will be more urgency..just keep in touch with them. They know its scary and they wouldn't put off something that 'needs' to be seen right away. This is totally precautionary.
BTW: arrhythmia's can come and go intermittently so your vet did not "miss" anything in earlier visits