Honestly, the way this reads to me is "My girlfriend and I went shopping and I bought a whole bunch of stuff I needed, then she wanted to look at stuff for her when she didn't really even need anything. Boohoo she's so materialistic and spendy." You don't even mention if she bought anything.
I could imagine her side of the story going something like this: "My boyfriend asked if I wanted to go shopping the other day. We spent time together picking out a bunch of clothes for him and he bought a shit ton of stuff. After that I wanted to spend some time looking at stuff for me but he looked at me and said 'what for?' He got really impatient and ticked off and accused me of being materialistic. I didn't even get anything. After I spent all that time looking at what he wanted to, he didn't want to spend any time looking at stuff for me. That made me feel like he doesn't care about me and he doesn't feel like I'm worth spending any time or money on. If he just wanted to go buy a bunch of clothes and leave as fast as possible, he shouldn't have invited me shopping. He should have just said 'I've gotta go buy a bunch of clothes, but I want to be as quick as possible and not spend a lot of time shopping, wanna come?'"
Now, that could be completely wrong. Maybe you spent hours shopping for her, maybe she was whipping out the plastic left and right to pay for all her purchases, but that doesn't really come across in what you said. I agree with other posters who are asking if she's spending your money or hers? I would add does she have consumer debt? And maybe: Is she wasting a lot of money while she has student loan debt? In my perspective I'd be concerned if she had consumer debt, concerned but to a lesser degree if she had other debt and was paying it down more slowly than she otherwise could (less concerned because maybe she just hasn't seen the light yet/ never considered another way), and would consider backing off some if she's 'wasting' her own money but still managing to add to her savings every month just at a slower rate than would otherwise be possible.
Have you two talked about money and your future goals? The kind of goals that cost money: house, wedding, kids, travel, (early?) retirement, college for kids, nice cars, private school, fancy clothes, paying off student loans? You would be surprised how many couples don't talk about money and finances. I think approaching the conversation from the perspective of what your goals are and how to achieve them is better than the how-you-spend-your-money-bothers-me approach. It will be far, far easier to agree to work toward your goals if you have the same goals. If what you want in life is a modest house, no kids, and to retire at 45 and she wants a big wedding, 3 kids, fancy cars, a country club membership, and to keep working until she's 75; then you are going to have a very difficult if not impossible time getting on the same page.