This can be a normal finding in dogs whereby the last voided faeces is more fluid and watery, so I wouldn't be worried. This is as long as Rosie is well happy bright alert and doesn't have diarrhoea, vomiting or any blood in the faeces. You could try bulking the faeces up a little with some bran (bran flake cereal is fine crumbled into the food, maybe 1/2 a tablespoon) Monitor her appetite if you feel it is definitely increased then she should be seen by your vet as sometimes conditions such as diabetes present like this, but I would expect her to be drinking excessively too!
Good luck and regards
I forgot to say, if you are concerned and do take Rosie to the vet, your vet may find it helpful if you take a faecal sample with you in case they wish to get it tested.