While officers can make contact and engage in "small talk" ("fish for information" as one poster put it), I see nothing that indicates that the officer developed further information which would have warranted extending the contact with the OP. (Granting the fact that I have only seen one side of the story.) It's very hard to make a case for reasonable suspicion, based on clearly articulable facts, when the articulable facts all consist of activities that are entirely legal.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.