Lots of things can make little ones act like this, worms or respiratory infections top the list.
Have they been exposed to plants or soil? If so, I'd administer Corid for coccidiosis. Blood in the poop only presents with a couple of strains of coccidiosis, not all strains; it kills chicks quickly, often before you realize you have a problem. Coccidiosis is virtually everywhere. Over time, chickens develop resistance to it, sadly though, chicks haven't had enough time to build up tolerance to it. Treating them for coccidiosis will NOT hurt them in any way, even if they're not infected, so I'd treat for that asap. In this case, prophylactic treatment is much better than not treating.
What type of feed? bedding? How is the watering situation? Any chance that anything is getting moldy? Link is website regarding fungal infections in chicks
https://www.dummies.com/home-garden...ngal-infections-in-chickens-molds-and-yeasts/
Cedar anything in their brooder is a no-no, causes respiratory distress.
Ensure that your chicks can get out from the heat source in your brooder too, fluffy feathers isn't a "cold" issue as much as it's a "I don't feel good" indicator. Too much heat can kill faster than not enough heat.
In no way am I saying that any of the above ideas is what is going on in your situation, simply trying to eliminate the question, answer, wait... question, answer, wait... question, answer, wait... chicks often don't have enough time to WAIT for a solution, so trying to get you some basic ideas and strategies so that you can, through your own observation of your chicks, determine what you think is their problem. I'll keep you and your feathered family in my prayers.
Pictures are always welcome, of the chicks, their housing, and their poop are all favorites ;p