fresh blood in 6-7 weekold chick poo

[Rhoise]

just finished(yesterday) 10 days durmycin-10 for respiritory symptoms/problems,(gurggling, raspy breathing, lethargic acting)....... gave them h20 with ACV and probiotics this morning(been on ACV since friday).....everyone seems healthy, has healthy poo as of yesterday, except today found two blood poos in run, and another fresh one(while my back was turned) from 1 of three birds....... checked everyone out thoughly no injurys, healthy looking butts, poop free butts, wondering is this just stressed gut linning from antibiotics or something else? I will continue to monitor......thoughts? greatly appreciated
Julie