Hi ChiL

Welcome to the NT forum. Hopefully others will come along and help with suggestions as to a possible cause.

From your description of symptoms one possibility could be a low Vit B12 level. I had numerous falls (along with other symptoms) up until 5 mths ago when I started supplementing with Methylcobalamin 1000 mg. Since starting I've only had one fall - like yours, mine were unexpected and sudden. There's a very comprehensive thread on B12 deficiency here:

http://neurotalk.psychcentral.com/thread85103.html

It's long so read MrsD's posts for relevant info.
If you think this might be a possibility and want to get tested read the info about testing, results ranges, treatment etc before you do. Especially about what number is too low - get a printout of test results.
Hope you find some answers - let us know how you get on.