It's a little more unusual for chicks fed with medicated feed to get it, but still possible. Keep doing what you're doing and let us know if another symptom shows up. Hopefully with the antibiotics and corid the problem will be resolved soon!
Pictures could help. I think it is most likely coccidiosis. Keep treating with Corid. Did you have your chicks vaccinated? And are you buying the medicated or unmedicated feed?