Aww poor sweet Luna. I was thinking the same thing as Randi, that maybe it could have been the milk.

I know my cats just lay around also when it is so hot.
We've been having high temperatures with the humity levels at 35-40C, but we do have an A/C in the bedroom, so when it is on, you can find them and the dog there.

I hope and pray that whatever it is Kirsten, that Luna will feel better soon and I hope you do to.