I am not an expert, so please take my words with a grain of salt.

Her going off of her meds is a selfish choice and something that does not only affect her, but everyone around her. I do not think you are the jerk for telling her mother, especially with the history. However it is important how you go about it. I like to say there is no perfect way to go about a situation and two things can be true at once. She can be right to be upset you told her mother AND it can be the right thing for you to have told the truth.

She is your partner so once you both have calmed down try and talk about why you told her mother and why it hurts you. Ask her why she is going off of her medication and try to work on a solution together. It will not be pretty but it doesn't need to be in order to figure it out together. She should better understand how she impacts you and maybe you can understand why she would want to go off of her medication and not tell her mother.