What are my rights while answering an investigation?

I was asked to be questioned on a open investigation. They said I could walk out at any time cause I was not being charged, but since I am on probation they made me be there. They needed a warrant for my DNA so I told them to do what they had to. So they did. They asked me if I had ever seen the vehicle in question. I said no. They said they found My DNA in the ride which was wrong because they did not have my DNA at the time. The sheriff on the case was trying to steam roll me for information I did not have they called me a liar. They said they found a cigarette in the car and a cup out side of it. I told them I work for a hotel and when I smoke the cigarette butts are left outside and lots of time I watch vagrants collect them then who knows where they go. Also, I now take them with me and no logger litter. The sheriff who served the warrant for my DNA was pumping me for information that I didn't have. He called me a liar and told me out loud with people around who was the jury going to believe me or a felon, he told me, so it doesn't matter what you say. I was upset and at my work of all places I worked there for four years I don't need them thinking less of me. This is a very scary ordeal and I was never even in the ride in question.