The Brevard County Sheriff's Office tells TMZ ... Nicky was arrested last week in Cape Canaveral, Florida after cops say she was giving her boyfriend oral sex while topless and in PLAIN VIEW. We're told multiple people witnessed it because they were in the parking lot of an apartment complex.

And get this -- cops say a witness confronted them and told them to beat it. That's when things allegedly got violent ... with Nicky and her bf ripping the witness' shirt and punching him in the chest and the back of the head with a closed fist.

Cops also say when they arrived, Nicky ran before she was caught and arrested for unnatural and lascivious acts in public, battery and exposure of sexual organs. She's out on bail.