Do you have a close relationship with your mother? Can you sit down and talk to her about this?
Unfortuately our addiction ends up affecting everyone in our family and it is affecting your mom as well in a different way. She could be just venting and it was the first thing that came out of her mouth. I think you need to give her a break. I would do my best to let this go if you possibly can. This is just my opinion though.