Why would my doctor perscribe me Gabapentin for a moving knee cap?

I have been having problems with my knee for a few months now. I went to the doctors a few weeks ago and they told me that it was the muscle and they gave me codene and put me on a waiting list for physio. When the codene was not working and the pain was getting worse I went back to the doctors and was told that my knee cap was moving as my thigh muscle was not strong enough to keep it in place. She gave me Gabapentin which when I got home I read up about them and they are used to treat Epilepsy. Why would my doctor give me these? Also is it likely that this is going to get any better or am I going to be stuck with it?