Miranda warnings apply to custodial interrogations conducted by the police or law enforcement. Private citizens including security officers do not need to read Miranda before questioning.
You said that one of the people from the *store* questioned you. The Miranda warnings typically only come into play when you're being interrogated by a police officer or government investigator - not by your boss or internal company security.
Miranda does not apply to private citizens being questioned by someone other that government agents. An argument may be made that loss prevention officers are acting with the authority and cooperation of the police, but that argument would likely fail.