My (24f) girlfriend (22f) of only 4 months made a copy of my house key and I'm creeped out

First-time poster, on mobile so sorry for format issues, etc.
I've been dating my current GF for about 4 months now and she's seemed really great so far, just a little clingy and overbearing but i found out literally 2 days ago that she has a copy of the key to my apartment which I did NOT make or give her.
A lil background: not long into the relationship (like 1-2 months) I had to go out of town for a couple days and needed someone to tend to my dog and GF insisted I let her do it so I did. That's the only time she could have made the copy which means she's had it for 2-3 MONTHS and hid it from me.
Anyway 2 days ago she met me after class and went home with me like always and when we went in the house, I noticed the dog's paws were dirty. He's strictly an indoor dog so I thouht that was weird and said so and GF just casually says "Sorry, probably got dirty when he got out this morning!" I questioned her about it and she just casually admitted that she came into my place after i left that morning and accidentally let the dog out. Is that creepy af or am I being sensitive? She told me about the key then got upset when I said it was weird and wouldn't give it back because "I might need her one day". We argued about it, I yelled and she left crying and now i feel guilty. I feel like maybe I was too harsh (I'm oversensitive about this stuff & I know it) but the idea of someone in my house without me knowing REALLY weirds me out. Am I being a jerk here or did she really overstep? And how do I handle it from here?
Tl;dr girlfriend made a copy of my house key and came into my place without permission and I don't know what to do about it


Source.

Well i didn't think it was to big of a deal till you said she was in your house without your knowledge. Damn right she overstepped. You don't just take it upon yourself to enter someone's home with a key you made behind their back!

Jeepers creepers