Was being stopped and questioned by a cop legal?

I went to a park near my house the other night at like 9 at night. after being there for only 5 min a cop showed up with a German Shepard blinding me with his light. According to him, the park was closed so we were technically trespassing. He checked my name and record and for 10-20 min he went on about how the lights besides illumination lights were on so it was closed and kept asking why we were there. he let me go finally and I had to rush home to beat curfew. But when I went back today, there was absolutely no indication that I could not be there. no hours posted, no fences closed, no trespassing signs, nothing but a much of baseball rules of conduct and no pets on playground. Is any of this legal or can it be called, unlawfully detained by law enforcement? And no I do not know many specifications on laws so don't call me an idiot in this question.