My bitch Cassie had her puppies 3 weeks ago and since the day prior to whelping she has had diarrhoea.  I initially put her diarrhoea down to her impending labour, everything relaxing and getting ready for the birth...
Also wondered if the hot weather was to blame?......

To be safe I took her to the vet and he said she seems well and gave us Milbemax incase it was a worming problem, and also Pro-Kolin+ (prebiotic) to settle her tummy.... 3 times daily since....but she still has diarrhoea.  The diarrhoea isn't particularly foul smelling, no blood or mucus and it literally washes away to nothing with a bucket of water.

Any ideas what could be causing this?  She does seem very well, no change in her appearance or nature and has taken to motherhood amazingly....but should I be worried.  Is it possible she could have damaged her bowel whilst whelping.....the pups were born about one and a half hours apart except for the last puppy who came as a surprise about 3 hours later!  Just a thought.

With such a long occurance I think you should take her back to the vet with a stool sample for analysis.

I agree that you need to take her back.  My bitch who whelped a week ago had bad diarrhoea for 2 or 3 days after whelping.  I knew it couldn't be a worm problem, as I Panacur from day 40.  But she had eaten all 7 afterbirths (too many for a small breed I think.)   My vet said it wouldn't hurt the pups by giving Pro-Kolin, and she only needed 3 doses.  Stools were completely normal after that.  I would have been back at the vet pronto if it had continued.