She may just have dry skin. You could try TMG ( I think that's the name) Oil in her tail. We use it on two horses at the barn I ride at and it helped allot. They stopped rubbing and have nice healthy looking tails.
If you do ALL of the above, and she is clean and well-oiled, and she is still rubbing her tail PAST her heat, it's probably worms. Don't be convinced that she is worm free bc of your drug rotation. Please read: Fighting the Dewormer Resistance - HorseChannel.com