<human>: Condense the text into a few key points

So I recently got the number of a nice, pretty girl who looked and sounded interested in getting to know each other. I asked her out to lunch, and she said no because, "I'm actually talking to a guy and I'm not sure how he would feel if we got lunch."

I took that to mean she's not interested. Fine. There're more girls out there.

But there was something tugging at the back of my head all day and I think I figured it out. 

Why would a guy that she's just "talking to," not even in a relationship with, be jealous over lunch with someone? So much that she's concerned about it? Regardless of my intentions (I didn't know about him before, but I would have been perfectly willing to keep it platonic if she asked), I think a big red flag went up in the back of my head

I think I've narrowed it down to two possibilities, which is why I'm posting here asking for advice. 

- She's got a clumsy way of saying she's not interested and my imagination is running wild

- It's a big motherfuckin red flag

I've had friends that were in abusive relationships and those weren't fun. But then again I also have so little to go on and I barely know her. Do I say something or do I just let this one go?
<bot>: Not sure if girl I asked out is headed down the wrong path. Do I say something or just let it go?