I really need help. I just got caught?

My boyfriend's mom just texted me and said she knew that we smoked pot. She said that either I could tell my mom or that she would. I'm just a teenager in high school, and I'm freaking out. I have a 3.9 GPA, it's not like I'm a bad kid. I just really need advice on what I should do, how I should tell my mom, it would be very much appreciated. And I know that some people think that smoking pot is a bad thing, so I don't need to be ridiculed.