Limb pain and fainting

My friend has been experiencing sudden fainting and afterwards waking up to having extreme pain in her shoulder. She's described the pain to be a burning sensation, which seems to spread into her arm. She is healthy, is not overweight and is almost 20 years old. She has gone to the doctor to get it checked out since it used to be a common problem (she says its when she was wearing a backpack and putting weight on that shoulder) but they couldn't find anything wrong and did nothing to help her.

She has no warning before fainting, no dizziness or feeling light headed. The only thing that we can think of that might have to do with her shoulder is that she hurt it a long time ago when throwing in soft ball.

Please help, we're afriad she might faint while driving or in other situations where it would be dangerous.