Blood and mucous in the faeces is generally indicative of colitis, that is, inflammation of the colon (lower intestine). This can be caused by parasites (worms), bacteria, allergies, diet or stress. Adult dogs should be wormed every 3 months, so if your dog isn't up to date, get yourself some wormer from the vet. Any change in diet should be done gradually over time - slowly introduce more of the new diet and cut back the old diet. Think about any stressful events or changes your dog may have been exposed to lately. If the blood and mucous continues, or your dog shows any signs of illness - lethargy, inappetance, diarrhoea, vomiting, you should seek veterinary attention.
I am going thru the same thing...I took my Frenchton to the vet with similar symptons and discovered he had Giardia...My advice would be take a stool sample to your vet so they can check for parasites..or God forbid something worse like Puppy Parvo...don't wait, the sooner the better..good luck with this ok...
My dog gets this when he's eaten something he's not supposed to. Is your dog ever around a cat litter box? My dog sneaks in the litter box, comes out with litter all over his nose and pretends like nothing happened. Then we get to go to the vet and get antibiotics for tummy issues. He's eaten several things that cut his insides, and he had to have antibiotics for those too.