Girl's Behavior

She Has A Boyfriend, But Showed Me A Lingerie Photo?

This girl I've been talking with for about a month (which I won't lie I like her a lot) has a boyfriend (I know..). I've never met him, but she brings him up now and then. She is a pretty reserved girl overall, she doesn't really put out or anything. I swear I'm getting mixed vibes from her. I feel like she is interested in me, but then other times she just acts like we're friends. I mean we're definitely AT MINIMUM friends. But she warmed up to me insanely fast. Her body language has become very open, and I'd say she acts very interested in me in person.

At one point, I asked about a tattoo on her lower arm. She showed it to me, and stuff and I showed interest. I asked if she had any more. (We were in a public place mind you).
She took off her sweater and showed off some other ones on her arms, and then at one point she pulled her shirt aside kinda secretively to show off one on her stomach.
Then she said "oh there's also one on my thigh". She pulls out her phone, and shows me a picture. It's her standing taking a selfie, in her underwear.

I could understand if she was the type of girl to put out.. but she's really not. I mean yeah, I'm sure she wanted an attention trip and to see my reaction.. but why? That's not something you just go and show someone you still really don't know. I can't say I didn't enjoy, but I'm just confused. It only confirms my suspicions that she is interested in me.
But then other days she'd mention her boyfriend on and off quite a bit, especially the past 2 days. It's almost like she's trying to get a rise out of me and see if I'm jealous. Normally I'd just think she's trying to hint like "I have a boyfriend screw off".. but NOTHING adds up.
Mixed feelings in her relationship? Likes me? Testing jealousy? Insecurities and seeing if I am loyal? I don't know.
She Has A Boyfriend, But Showed Me A Lingerie Photo?
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