How to find if a model was saved from Django Admin, or elsewhere
Since you don't need to create the related model thanks to the presence of the InlineAdmin perhaps it would be better for you to remove the post_save signal receiver rather than adding more code inside post_save to determine where the save originated from. 
That way you can just place a call to create_related_models in your serializer's create method to achieve the same objective.
Is there a way to identify if the current save() or post_save happened via Django Admin? I want to do something differently for models saved via Admin, than models saved from my Django Rest Framework API.
Specifically, I have a create_related_objects I call on post_save of model Transaction, to create related object CalendarInfo.
But when creating Transaction via Admin, because the related CalendarInfo object is an inline, I don't need to call that create_related_objects on the Transaction model's post_save.