S.R.
She's lucky to have you. I agree that you need to get some kind of professional help - the sooner the better for her sake. You may have some difficulty if the mother (or father) won't at least agree to let you take her to a doctor. I don't know what the legal requirements on the doctor are for obtaining parental consent to see a child.
When my 5 year old was having trouble with social skills, self-confidence and shyness, I learned she struggles with anxiety. Her biofeedback therapist explained to me that when you see a kid over-reacting significantly to things, you should suspect anxiety. If they are already stressed out (too much adrenaline), then the least frustration is just too much to bear. Biofeedback (not invasive, not painful, really just breathing and muscle relaxation techniques) was very helpful for my daughter, and can be done with kids as young as 4 or 5. That might be helpful AFTER you discover the cause of this change in behavior.
The recommendation of a comprehensive neuro-psych evaluation was a fantastic one.
We love Rusty Lozano (www.onlinebiofeedback.com) for the kids' various issues. He does play therapy, family therapy, biofeedback, etc. He has 4 kids of his own, and his wife is also a child psychologist, so he is very empathetic. If your granddaughter is afraid of men, he could recommend a female therapist for you.
Good luck.