involuntary arm twitch like wave.

Liv is 1 yr 3 months old, we've only had her 6 weeks.  only in the last couple of days she will be sitting there normal and does an involuntary wave. doesn't seem to bother her.  but it only just started.

I have never heard of anything exactly like this.  I have one cat whose back end twitches sometimes when he's laying up on my shoulders (front legs hanging off one shoulder, back legs hanging off the other shoulder)  I mentioned it to the Vet, because it's very obvious when he does it, but she didn't seem to think it was anything to worry about. 

If you are concerned, then perhaps you should consult with your Vet