Questions about how it’s contracted

So, my new gf got scabies and supposedly she had it for the whole 2 months we have been talking. She did itch and have random bumps come up for 2 months but tried to get doctors opinion but they said it was nothing. She finally went to dermatologist and they said infestation of bugs. It’s scabies and I now have it because she slept at my house in my bed for weeks and we had sex a lot. Anyways, was just wondering how she could have got it? She hangs out with 2 people me and her friend. Her friend had sex with guy for money way back like a year ago, and they like sit together but no like holding hands or skin to skin for long periods of time. And obviously it wasn’t from me. So I’m just confused on how she could have got it? She’s a hairdresser but I heard you can’t get scabies from that. And I really don’t take her as the girl to cheat like she doesn’t leave her house and we Snapchat all day. But idk it’s so weird and doesn’t make ANY sense. Anything helps. Thanks.