Interpretation of the beginning (spoilers)
Here are the oddities, I think, at the beginning. I think they have a lot more to do with what's goin on than the ending, or the weird middle.
She has too many tracer marks to have just started treatment. The day before this, she had been consoled by the older doctor for having been diagnosed (which inicates it is a recent event). The timeline is ambiguous as to when these things started.
Her phone call with her husband, is very odd/hallucinagenic. Also, at this time she seems to react to him joking about being around her sister, and about inviting her sister along on a just-them Vegas trip. This is the first note about her sister, and includes hints about the three-way relationship.
The patient she reviews is not guarded/controlled at all, and is in street clothes. Also, her questions are f'd up, as is indicated by the older doctor the next day.
She hears sounds simliar to the "mice" as early as the diner scene, as well as rhythmic sounds (clock, clinking the coin).