How is your pup today?
You are going through what we went through with our border collie pup - it can be a bit distressing watching them poo down to the mucus then blood. As the vet has probably told you, as long as the blood is red its not so much of a problem (although not great!). It means that your pup has strained and burst a blood vessel. If the poo turns black it means there is old blood in there and something different / worse is going on.
A rule of thumb regarding dog food -- if you can buy it in Tesco its unlikely to be good for the dog. Dogs love Bakers cause it is full of sugar and colouring, like smarties before they took out all the E numbers.
Chappie didn't work for our dog.
What does work is a bit labour intensive, but totally worth it, as our bcollie still gets an upset tummy at times.
When the runny poos start we starve him for 24 hrs (yours is just a wee pup so 12 hours is about right) to allow the stomach time to empty itself. Then we feed him on a bland diet until the poos harden - this is overly boiled rice (half an hour in a simmering pan) this takes the starch out the rice and makes it look really nasty - like the gloop in oliver twist. And we mix through that a protein like chicken, tinned tuna, or scrambled egg (whatever we have an excess of in the cupboard). Once poos are back to normal, start adding in the kibble over a couple of meals until you are completely back onto a normal diet.
We couldn't get a kibble to settle with our bcollie, so ended up feeding him Royal Canin Vet diet (we were at our wits end and thought we would be cooking rice for him forever) - its really really expensive but properly hypoallergenic. After getting him settled on that for a few months we were then able to introduce a much cheaper (but still good kibble).
If you can get your pup onto a kibble that works don't change it!