Can my former employer really pursue a legal course of action against me?

I left the company I worked for 7 months ago to pursue a new career. Today,I received a certified letter from my former employer's Attorney. The letter states that I have been making "disbaraging comments about the company to current customers and their employees. Demand is hereby made to cease and desist making disparaging comments about the company to any third parties, including current clients and their employees or risk liability for defamation and other legal causes of action". Boy, was I shocked. Outside of voicing any complaints to my husband and my best friend, I don't talk about my former employer to anyone else. I think I'm allowed to talk to them about my work day. But I have always talked very highly about the Medical Director and the products to anyone who has asked me including my husband and best friend. Up until this letter, I have been ordering products for my own use as well as for my family. This letter now says I can no longer purchase products from them. Can my former employer really purse a legal course of action against me?