First off, I don't mean to bash your girlfriend. But you need to hear this.
I have to tell you, that she would go into a stranger's house and take off her shirt is unacceptable relationship behavior. No, I mean REALLY unacceptable, I really don't think you quite grasp the unacceptableness of such an action. I mean, it's not like she had an undershirt on, right? Besides all that, being a girl and going into a stranger's house and taking off your shirt is not only unacceptable, but stupid and foolish. I'm going to venture that she is young. Very young. 18 perhaps? 19? Less? For all she knows that man could have jumped on her. Doing such things shows such a complete lack of regard for personal safety that I feel it may indicate that she is very depressed and desperate for relief of circumstances she finds unbearable. Has something traumatic happened to her recently? Because it sounds as if she is not only questioning life, but everything she is as well.
Second of all she obviously has no idea that any decent guy who has reached age 26 does not ask teenage girls to come into his apartment and take off their shirts, for art or otherwise. She is not mature enough to recognize this as the pickup that it is and if she does she's immature enough to think it's cute.
I also want to add that if I were in a serious, committed, manogomus relationship with a guy, if he went into some random chick's house ands took off his shirt for her her, something like that would be grounds for instant breakup or at least serious separation.
Now would I, a single woman in her mid twenty's go into a random stranger's house and take off my shirt? Yeah, probably. But you must realize that this is because I have a serious slutty side which stems from extreme emotional and psychological issues. Normal, rational, well adjusted people don't do that sort of thing. When I was younger, rational and semi-normal I would have blanched at something like that.
This sounds to me as if she is disinterested in you and may want to see other guys, another indication of her shirtless stranger rendezvous. Although, I'll be quite honest, she may not know what she wants and if you break up with her and leave her to find her way and give her some space, she will probably call you within the week begging for you back. This is because most women are creatures strictly ruled by emotion which changes from hour to hour. And younger women are not only emotional, but confused as well. Trust me, we fuck around with men's heads because we ourselves are enigmatic emotional roller coasters. Common sense only prevails years later when we are able to look back on our actions without the bias of emotion.
I'm not at all saying your girl is a bad person, she's just a chick. Most of us chicks are like that, kudos to anyone who isn't.
Oh well. I wish you, your girlfriend and your relationship all the best. You sound very nice, which is a rare thing now a days and I hope things go well for you and I hope you never lose your niceness. I lost mine, and I can tell you it's not really something you ever really recover.