In the office, Jane tells Joan that she has to have her rings resized because she keeps losing weight.  At the Derby party, she doesn't eat anything and claims that is why she is drunk.  I believe that we will find out that Jane is suffering from anorexia.

Maybe.  On the other hand, she could have just said that to needle Joan about the obvious difference in their body types.  Especially because Jane's fashion choice that day seemed to mimic Audrey Hepburn, another ectomorph.  Just another subtle way of getting into Joan's face and flaunting her triumph.

I agree with both of you, but tend to think she was needling Joan.

Most likely she was needling Joan, Jane did plan on eating but she was too drunk to serve herself, an anorexic would not have thought to eat.

Maybe Jane is wasting away due to some form of undiagnosed cancer?  Perhaps ovarian?