Mother might be involved because your exgf has turned her mother into a codie. You can see the manipulation in this, right?
The second highly probable situation is that your exgf's mom has problems, maybe alcoholism or addiction issues of her own.
Mom has no business calling you in that impolite manner.
And, she is lying if she insists that you called when you KNOW you didn't.
She has no business involving herself in your relationship issues. That is none of her business.
She sounds like toxic person #2 that you need to go no-contact with!
You are doing really well, but all this checking up is not any good for you.
My husband never checked up on me when my drinking was at my worst. He already knew that my life had become unmanageable and he had enough confidence in his own good judgement to know he didn't need to become a detective.
You have made really good decisions here; most people would not need hyper validation, but the codie mentality will always have you doubting your own best decisions. You've done good, you have no reason to doubt your judgement.