I think the only way forward I would like is to discuss the situation, tell her you perceive her as changing and ask her has she and why, ask her if you did something/if she is mad at you, etc.

I had a roommate like that, that I got along well with and then she suddenly got snarly and it was because of something someone else had said about me, some things she had taken the wrong way, etc. You do not know what happened that weekend you went away, who said what about you or you and her relationship or what went on but obviously something did? If you can calmly explain how puzzled you are at the change you feel happened and wondering about it, etc. without being blaming, maybe you will learn someone else said something or one of your other friends said/did something or someone knows someone who knows someone, that sort of thing and you can at least know what you are up against and what has happened? Maybe things will get a bit better/less awkward and maybe not but some of the feeling of uncertainty will at least be gone so you can feel more neutral?