prospective employer called current employer and made bad comments about me

I am currently employed but my hours have not been consistent so I have been interviewing for other positions. I had an interview recently and I was asked if the potential employer could contact my current employer. I said yes because I thought if I said no it would look bad. She asked why I wanted to leave my current place of employment and I explained that I was frustrated with the lack of consistent hours. I do not dislike my current job or co workers, I like them.

The prospective employer called my current employer and told the I was applying for a new job and that I was very unhappy with the current job.

It turns out that I did not get fired, I got more hours at my current job. However, I am very angry that the prospective employer did this. I would like to contact a lawyer and have them write a strong letter telling the employer that there actions were illegal? And had I lost my job I could have sued. Please give me any feedback on the situation. Thank you!

In addition, I never gave the prospective employer a reference list.