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

I have been seeing/dating a girl for almost a month.  We are both 24.

She is great- always a fun time together, super pretty, awesome sex.  I was having fun until recently.  We agreed to become exclusive and she started calling her boyfriend... and now, I'm freaking out a little and I'm not sure why.

I could have seen this heading towards a relationship, and I did, but for some reason for the last 24 hours there has been a sinking feeling in my stomach because I know that if I feel this way now, it's only going to get worse, so I'm going to have to hurt her eventually by breaking things off.  

I don't know if I should give this feeling a little time to pass - is it natural?  I'm used to being single and I like being single.  But I also like her.  I feel guilty and terrible that I'm not brimming with joy that she's my girlfriend.  It's just such a change, and to be honest, the label of 'boyfriend/girlfriend' came a little too soon.   I did not mind being exclusive, but somehow now it feels like a serious, long-term relationship, and I just can't get down with that; I've only known her for a month. For the record, this is not about seeing other girls.  It's about this trapped and guilty feeling.

I don't want to lose her, but of course I will have to if this feeling persists.  I can't tell if I'm just overthinking things too much, or if this is a warning that I need to end it as soon as possible.

Can anyone help me out or offer advice?
<bot>: was feeling fine in new relationship, but suddenly feeling trapped and like I need to end it.  Wondering if anyone else has experienced a similar feeling ~1 month in to a new relationship.