W.H.
THis happened to my mom about 15 years ago and was also a benign tumor (not cancerous) - she had a partial hysterectomy with that so the recovery was a little longer than we had originally anticipated.
There are so many possible reasons for this happening and the best part is that they are going to biopsy right then and there. If they have to remove anything extra, they will already be in her rather than scheduling another procedure. These growths can happen at any time for no reason. Her and I both have very cystic breasts as well, so, we get checked regularly to make sure there is nothing else going on. I can't tell you not to worry about your mom because it's going to happen anyway. Just take deep breaths and think wonderful thoughts of her and all the cool things you'll do when she's recovered from this.
I'll say a prayer for her tonight too.