Overtime

Tell me if I'm wrong: I had a hard day and as I was leaving this girl approached me.
"Excuse me? Miss?"
"Oh I'm sorry, I'm not on the clock."
"I know you're not on the clock, but I just need a female's opinion, so if you don't mind..."
"I do mind. I'm sorry, but I do this all day. I have to go."
She had plently of opportunities to ask me for my opinion, because I walked past her a few times. Also, there were a few other female employees who were still on the clock. And, I used my nice customer service voice.
When I mentioned to my coworker and manager what words me and that girl exchanged, my manager said I should be nicer. He said karma is a bitch and there is no reason to act like "them". I said that I really don't give a fuck (I know, that part isn't nice), and that I work all day and I'm allowed to assert myself. The time I have after work is my time and I absolutely don't need to spend it helping a customer. Perhaps he thought I used my aggressive voice.
On the upside, I get to come in fifteen minutes late tomorrow because I got overtime this week. And they don't want to pay me for overtime.