<human>: Can you condense the text into a few key points?

I am in my mid-twenties and so is my girlfriend.  We have been dating for a solid four months and a few days ago something very peculiar happened.

The situation is this, she texted me what I was doing that day at around 4pm while she was at work and I told her I was at home drinking and relaxing (I work in retail, christmas is hell, you understand).  So I text her a few times to no avail after she's out of work, and then I get a text saying she just went to a band's practice session.  She has no car and only uses public transportation.  I asked her how she got there and who she knows in the band, and she says she knows a guy who is in the band.  

Naturally, because of our past and how we managed to be together through promiscuity on a few occasions, I was worried.  We exchanged some other texts before the day was done.  This is where things get dicey, because over the next few days she tells me she actually spent the night at said guy's house with his band mates and also finished an entire bottle of wine on her own, making her drunk at a guy's house I've never met nor known.  Spending the night, she is then driven to work the next day and her co-workers apparently notice that she looks a bit ragged and ask her how her night was.  

So, this is where I am.  I ask /relationships/ in an effort to determine how I should feel about all of this.  Does it sound oddly curious?  Should I bring down the hammer?  Is this commonplace with couples in some kind of connection I am unaware of?  Should I worry if she cheated on me, or is that too irrational?

Just tell it to me straight because I have been racking myself over this for days and been soothing this burn the best I can.
<bot>: How should I feel if my current girlfriend of a couple months goes to a boy's house I've never met, gets drunk and spends the night while I hear about all of it days after the fact it happened?