I daresay you've already checked out the signs and symptoms of Trigeminal Neuralgia (TN).... that being a common cause for facial pain. If not we have a forum here on NeuroTalk for people who suffer from TN, and there's lots of information available on the Internet. Here's the direct link to our TN forum : http://neurotalk.psychcentral.com/forum26.html

I don't know about the rash that you had, but intermittent sharp pain is characteristic of TN. Here's one site that has some good information. http://www.mydr.com.au/pain/trigeminal-neuralgia

Temporal Arteritis (TA) is another condition causing facial pain that comes to mind. It's a common form of systemic vasculopathy affecting patients older than 50 years.

Wikipedia have a good explanation giving symptoms and even a diagram. Here's the link below for you to check out and see if they might apply to you.
http://en.wikipedia.org/wiki/Giant_cell_arteritis