It's possible for them to eat it too quickly, and the food can expand in their stomachs. You could try soaking the flake in a bit of aquarium water, before feeding it to them.
no i am pretty sure its not a infection because it appeared after fgeeding and was not there before and some of them were eating flakes as big as their bodys if its just food in their stomach it won't harm them will it?
Well, yeah, I've seen it listed as possible causes of constipation, and possibly other internal problems.
I'd keep an eye on 'em, I imagine it sh. go away soon, if it's not a problem. Maybe try breaking it into smaller pieces? and/or soaking it.