That was very unprofessional of that nurse...she should have NOT mentioned something like that before the dr. has a chance to discuss it with you. But what's done is done...she was just flat wrong. I'm thinking that, like Straydog said, this may be something insignificant since the dr. hasn't talked with you or scheduled an appt. with you. So, with that in mind, I think you should stay calm and not worry. There actually are many things in our brain that can be described as "lesions" that aren't a danger to us....so I'm crossing my fingers for you.
Your situation reminds me of something I went through, several years ago. I had an MRI of my brain because I was having severe headaches. So it was Friday evening and I got home from work, turned on my answering machine to hear my dr.'s voice saying: "You have a tumor in your maxillary sinus cavity, so I am sending you to a neurosurgeon...Call my office to discuss further." Well I was terribly worried over the sound of "tumor", so I immediately tried to contact my dr. She wasn't on duty and the on-call dr. knew nothing of my case. I had to wait the entire weekend before I could finally reach her later in the day on Monday. Anyway, come to find out, it was actually just a nasal polyp. So all that worrying for nothing.
Straydog is right. This kind of stress is really bad on your body. Especially with the diagnoses you have. Please just try and occupy your mind by doing things you enjoy, so you don't stress about this. I hope everything turns out ok for you. Please let us know when you find out.