Jasper has been biting and scratching his behind like mad for the past week. He went to the vet yesterday and he said there was some flea dirt but couldn't find any fleas. I gave him Advantix yesterday afternoon and he's still at it, biting and scratching. Could it be the flea dirt that's irritating him? The Advantix? A reaction to grass?? Highly grateful for any ideas - even the Vet doesn't know!