While in some aspects the defintion of arrest is not an academic one, here are my thoughts:
By the officer, no. Unless the officer said, "You are under arrest", you were not, since the immediate charging of a crime had to follow.
IF your state permits a citizen's arrest for NON felonies, no to the merchant also.
Why:
1. If you were told you were under citizens arrest, then the law will require the deliverance of your person to a person authorized to take you into custody, which you were not by the officer; OR delivering you themselves to a custodial house, jail, etc., so NO arrest took place.
Even if the law permits citizens arrests as described, merchants I am sure, to do NOT authorize thier agents to do so, merely detaining them under the shoflifting laws suffices without possibly complicating the law and liability for unlawful arrest, false imprisonment etc.

