My question involves police conduct in the State of: Illinos

Need some help with a local Police Officer who continues to stop me. My sister-in-law
did something to be arrested (I have nothing to do with her, live several miles away).
The Officer has stopped me and 'quizzed' me about where I was that day. I don't even have
a cell phone. I am handicapped/use a cane. The Officer sometimes is aggressive, sometimes
he is friendly.

The last time I saw him, I asked him why he was stopping me. And that I didn't want to
be questioned anymore. He threatned to arrest me. When I asked why, he did not give an
answer.

What am I supposed to do? This is very upsetting.