Paralysed Guppy?

Not much recently, except today I noticed that the Guppy 'Blackie' seems to be paralysed from the waist down. She's swimming using just her pectoral fins, which doesn't look good. Her back has suddenly arched and she's lost her 'pregnant' look and her colouring has gone slightly.

It's all come on very sharply - I suppose it doesn't look too good.