This evening my seven year old spayed bitch had a very stiff, rigid tail. It was swished to one side like she was flagging. It was trembling the full length (the tail isn't docked). I stroked her tail and gently pulled it straight but it went straight back to its original position and was still trembling.
I've never seen this before in her. Should i be worried? It's not a sign of a stroke or something is it?

Google the following terms:   limp tail, dead tail, cold tail or swimmers tail

One of my dogs (HWHV) has had this three times now and he's not 2 years old yet.  Neither of my other dogs have ever had it.  I give Metacam for a couple of days and keep him on house rest - no walking - til it picks up again.

Check with your vet though if you're worried or if it doesn't match the above. 

Thank you for your reply dogs a babe.
The symptoms on the limp tail, swimmers tail etc don't match what her tail was like at all.
All day today she has been absolutely fine, her mood, appetite and general silliness. Apart from the fright it gave me at the time i hope it was just a peculiar one off. She is a timid natured dog so maybe something spooked her.
Either way thank you very much :-)

Could she have jarred her spine making the muscles in her back going into spasm and pulling her tail to one side? Was it painful/difficult when you tried to straighten it?

Hopefully it was just a one off

I think you have hit the nail on the head :-)
She had been asleep in her bean bag bed. She got up after i had called all the dogs to go do their business. It was when she stood up from being asleep the tail looked strange. Perhaps she had slept or laid on it funny.