There are lots of things that can cause this, so it is good you are going through the testing. We saw diarrhea and blood in my daughter's diapers at 4 months, and after testing and a GI specialist, we believe it was a reaction to her 4 month vaccines. She is fine now, but it took a while to clear up, and we continued to vaccinate her.
I know it is scary, but probably is not a serious condition.
Let me know if you hear more/discuss.