Mm - if that was mine (and I do mean mine) I would be worried. It could be a tumour which would explain the bleeding of a friable mass, but it could also be caused by rubbing by dags, except the dags don't look bad enough. Have you noticed her rubbing her backside against a fence or post? A persistant itch could be caused by intestinal worms and make her rub herself so much she has caused damage. It's the wrong season for reversing into a nettle patch. If it doesn't settle quickly, ie shrink back to normal size, shape and colour, I think you should send the pic to your vet - or maybe just do it anyway then you can tell me I'm being alarmist.