I would like to know if I have a defamation case against my boss and co-worker?

I would like to know if I have a defamation case against my boss and co-worker? I recently requested for time off due to my newborn baby not having a baby sitter. I submitted my request in writing and while waiting for approval, my sister whom I work with came to me and showed me a series of text messages that was accidentally sent to her from our boss and a colleague.

The text messages was about me. My boss and colleague was discussing about my request for time off while talking crap about me in between, they even said "well if don't get time off, she might just injure herself while working because of her being a fatass and then file a claim"....

I know I gained some weight, but I am not fat. Anyway, I believe that both of them realized that they sent these terrible messages to my sister, because my colleague asked my sister if she showed me the text messages yet. When my sister said yes, they approved my request for time off.