Staffordshire Terrier paralysis and twitch

Hi there. I have an 11 year old staffordshire terrier. Over the last few months she has developed a sudden onset paralysis in her hind legs. When it happens she clearly seems upset and in pain, she doesn't make a noise, but it is clear from looking at her that she is very uncomfortable. She tends to fall down and sometimes starts to twitch and muscle spasm. It started off only lasting for a few minutes and she would be back up walking again, but now it seems to be lasting progressively longer before she can move.

The vet has said it is likely arthritis and has been giving her anti inflam shots, but it would appear with the twitch etc, that it is more than that. The problem is she seems to miraculously get better the second you walk into the vet surgery, thus making it difficult for the vet to diagnose...

Anyone have any idea?