I know that JW elders think they are exempt from adhering to people's rights, you are an adult subject to an expectation to privacy.
I suspect your mother made-up the story but if the elder's did divulge that information without an official announcement, they need to be taken to task legally over discussing private and disparaging information with others which includes your mother. They have no cause other than hearsay as they haven't spoken to you on the matter.
In court, they claim clergy privilege over not providing private information to the court even when they discuss private details to other elders, the CO and WT.
The change from announcing to the congregation the specific reason for disfellowshipping, to announcing 'conduct unbecoming a christian' and then changing to 'so-and-so is no longer considered a JW', is not without reason.
Don't be down but become angry and have your say through a lawyer over how your privacy and good name are being trampled-on and disregarded.
Perhaps if you mention to your mother that you plan to take legal action against the elders, she will fess-up that she made-up the story.