dogs upeset tummy?

My 2 year old dog had puppies in October and now for the past 2 months give or take she's had diarrhea, sometimes with blood. NO worms or anything like that are in it, but it's always diarrhea. I have two other dogs who aren't sick at all. She doesn't have any shots or anything so taking her to the vet would be very pricey and a huge deal. So I am wondering if anyone here has any info or tips that might help.. Thanks