my dog is about 15 years old,we rescued her 6 years ago,,,she is a 6 pound poodle mix,,,a few weeks ago while i was petting her she started to shake her leg something the she never used to do,,,now i just have to touch her and her leg goes crazy,,,i know dogs do shake there leg sometimes when you get the right spot,,but my dog would never do that,,,should i worry?