Does your cat stand with her back end near to a vertical surface for instance when she comes inside?  I don't know why she would do it at feeding time but I've seen this kind of tail shaking among cats (often female) as though they were going to spray (mark their territory) but they don't actually do it.

Cats who do mark their territory usually make the tail shaking gesture as well.  For those that don't, just the tail shake remains.  Cats I've owned have done this and in fact this includes my current cat.  Not all the time, just occasionally.

Maybe at meal times it's a mix of dominant marking behaviour and excitement about the meal.

My cats (7years old and 2 x 2 years old) don't shiver their tails. But I google a bit and found this site. It might help.

http://voices.yahoo.com/your-cats-tail-movements-does-mean-1969117.html