If it were glaucoma, (a) the pain generally gets worse and worse, and (b) her ophthalmologist would have detected it.
There are a lot of weird things that can cause eye pain, and the fact that her eyes are being checked and given the OK by a real eye doc should be reassuring. He/she would have picked up anything bad that's related to the eye.
However, since this pain always occurs behind the left eye, you might want to take a look at this website:
http://www.just-health.net/Pain-Behind-Left-Eye.html
I have experienced positive dysphotopsia and had posterior capsule opacification which was treated with YAG. Neither the dysphotopsia nor the opacification caused pain. I have also done extensive research on both conditions and never saw mention of associated pain.