It's so hard to believe that a psychiatrist wouldn't know this but I've come across other ridiculous situations with doctors so nothing is too outrageous it seems.

For some of the narcissistic ones, I wonder if they have to deny whatever it is the patient brought to them as in their mind, patients are not 'allowed' to know more than they do, thus devalued, seen as useless, stupid, ignorant, whatever it takes for them to maintain their self image of being bigger, smarter, more important.

It's like a mental block in their brain that a patient can question something related to their 'treatment'. Or because she didn't warn about weight gain, and instead you pointed it out, her mind blocked the fact it is true. Her defenses become the source of her need to gaslight you.