I know how scary this is when you see blood for the first time - so take a deap breath my boy had it too - turned out he has food alergies which irritateds his large colon beyond belief.
The symptoms could be many things ranging from giardia to worm - one thing is for sure this will not resolve itself and only a VET can determine how to treat it.
So, call your Vet, take the first available an appointment and bring a vecal sample.
Goosd luck and plenty of huggs ands kisses to your baby.