Likely so. How serious depends on the specifics. I don't blame you for being upset. Can you find out how much was shared? Why it happened? Lawyers make mistakes, but confidentiality of your data is very important. If the situation is very serious you can consider reporting what happened to your state's disciplinary board. Hopefully this is a one time mistake from which the lawyer will learn a lesson.
I am sorry this happened to you.
I am licensed in Pennsylvania. Members of my firm are licensed in various states, including Pennsylvania, New Jersey and New York. We handle cases involving personal injury (car accidents slip and falls, etc.,) medical malpractice, nursing home abuse, workers' compensation, social security disability and legal malpractice. Nothing I write on Avvo is legal advice, but instead contains general educational information. Please do not act or refrain from acting based upon what you read in anything I write on Avvo without retaining your own lawyer in your state. Also please remember that this post does not form an attorney/client relationship between you and me. If you have specific legal questions, you should contact an attorney in your state for assistance.
Yes, an attorney's unauthorized disclosure of confidential information violates the attorney/client privilege. The primary purpose of that privilege is to protect confidential information from your opponents in litigation, and there is extensive case law regarding how to remedy an inadvertent violation.
But while it is unfortunate that this happened, and while we all strive to protect our clients' confidences, these things happen. (That's why there's a large body of case law on how to deal with them.) It sounds as though your information was produced to another client rather than an opponent. Unless you were damaged, and unless the disclosure was intentional or reckless, your best bet is just to get the information back and move on.