my 16 week old puppy, border terrier X jack russell has a problem pooing, when she poos , she is obviously experiencing discomfort, she does a bit and then shoots off a few metres looking stressed, and does some more and then the same again until she has finished, vet has no idea but gave the usual antibiotics and anti inflammatries, after a week there has been no change, poo is not hard, no blood and looks healthy,and apart from this she is full of life, was feeding her on dry mix, but have now swapped to tuna and rice and veg, anyone any ideas

I HAVE A BULLDOG AND THE BREEDER RECOMMENDED BRAN FLAKES IF THEY ARE CONSTIPATED OR HAVE DIAHERREA (DONT KNOW HOW TO SPELL THAT) I HAVE USED THIS WHEN MY LITTLE ONE WAS CONSTIPATED AND FEED SMALL AMOUNTS 4 TIMES A DAY (HES 15WEKKS TODAY) AND HE WAS FINE WITHING 2 DAYS. TRY TO FEED THINGS THAT ARE BLAND BOLIED CHICKEN AND RICE AND TUNA AND THAT ARE A BIT RICH

It is not generally recommended to feed tuna more than a couple of times a week because of the dangers of mercury traces.  I'm not sure this sounds like a fully balanced diet anyway.  You could try a complete that has a bit more fibre to it, if you want to feed a home made diet I suspect you will have to do a lot more research to ensure what you are feeding meets all your pups growing requirements.
Has the vet examined the bottom carefully for something like an anal fissure?