<human>: Give me the gist of the text in a nutshell

Condensed version - I [20M] have been official with girlfriend [21F] for three months, but we dated for several months before that.  And when I say dated, I mean spent pretty much every day with.  However ever since we became official, things have changed.

Before we became an official couple, she was sentimental and 'couple-y'.  We'd say cheesy dumb things to each other, and were very affectionate, which I love.  However, when we became official, within a matter of a week or two this all evaporated.  I'll try to say something VAGUELY sentimental, about how good she looks today or about how I care about her and she'll immediately respond with a condescending snort and a 'Ok, whatever 1115.'

I believe that communication is a huge factor in relationships, so I've tried to bring this up - twice.  Both times, she's become immediately angry with me and accused me of trying to change her...  I don't want to change her, I just want to know what changed to make her so distant.  I'm experienced in relationships, I know a honeymoon period is common, but this is very different.  She refuses to acknowledge my question and I'm unsure of what to do - both times I've brought it up she's become angry and a big fight has resulted, ending in us not talking for several days.

There has been no trauma or significant change in her life in the past few months.  I just want at least to know what made this change happen.  Of course, it'd also be nice if she was affectionate again...
<bot>: My girlfriend used to be affectionate, but she isn't anymore; if I bring it up she gets absurdly angry and frustrated.  How do I talk to her about how this is an issue for me?