Sounds like coccidiosis. You need concentrated Amprollium (same ingredient that's in medicated feed), preferably Corid. Sulmet can also work I think, but is harder on little chicks - Corid won't hurt them, even if they don't need it. You need to treat all your birds if some of them are showing these symptoms. Even if you're feeding medicated, outbreaks can still occur and the medicated feed is not strong enough to treat the birds.
I found Corid in the cattle section of my local farm store (it can be tricky to find, it's used for a lot of different animals). I think I used 2 tsp per gallon of H2O but there are threads on here that can tell you for sure. Please get some Corid asap, as if you're seeing bloody poop it may already be too late. Also, avoid giving them any yogurt or high-protein treats as that can make the infection worse.
Where do you live? If you happen to be in Colorado (Denver/Boulder) I have a huge jug of Corid and would be happy to share. Good luck to you and your chicks!