What do I do?

I've been talking to this girl for a month now. We've had 3 dates and see each other at university twice a week. During those 2 days we walk to class together, hangout during our breaks, and walk back together. We've been romantic but haven't been intimate yet (she's only come over twice for a short period of time)

We went out this past Friday and it was great. At the end of the night she initiated more kissing and it was great. Over this weekend though she became more dry over text, initiated conversations less, if at all, and just didn't seem interested. Today is Monday and she has barely talked to me, left me on delivered for long periods of time (even though she's using Snapchat) and just seems completely uninterested.

Mind you literally a week ago she was so enthusiastic and engaged in our conversation. And now nothing.

Firstly, I have no idea what to do. I like her so much and just don't understand how she can flip that easily. She's been such a big part of my routine this past month with walking to school and such and I don't know how to go about this. Do I even bother asking her to keep up with what we usually do?

Secondly, why would she do this?

Any advice helps. Thank you