It sures sounds like a custodial interview to me..if a cop tells me I'm not free to go and begins questioning me..anything I say is free info for them to use against me. I'd want a lawyer I believe. It's a moot point now, but I would've been interested to see what they would have done if you'd said that. Of course, you'd likely end up stuck for the day.

From a law firm's site, (Not to educate the OP, but to educate the rest of us if faced with something similar):

Types of Police Interrogations
An interrogation can occur at the police station, in jail or at the scene of a crime. There are two types of police interrogations:

■Custodial interrogation - A custodial interrogation is an interrogation of a person in custody who is reasonably suspected of being directly involved in or responsible for an offense. The person being interrogated is not free to leave police custody. Once a person is in police custody, the suspect must be read his Miranda rights if the police want to question him and to use the answers as evidence at trial.
■Non custodial interrogation (also called an interview) - A non custodial interrogation is the gathering of information by police from a person that is not yet officially considered a suspect for the offense being investigated. An interviewee is not in police custody and is free to leave at any time. A non custodial interview does not require the police to read the suspect his Miranda rights in order to use statements as evidence at trial.
How to End a Police Interrogation
A non custodial interrogation can be ended by leaving. If the police do not allow the person to leave, then the interrogation has changed from a non custodial interrogation to a custodial interrogation. A custodial police interrogation may be stopped by:

■A clear request for an attorney
■A clear request to remain silent
But after either request, if the suspect initiates conversation, then any statements made may be used against the suspect as evidence at trial.