Crumble vs Pellet won't make a difference ... What were you originally feeding, name of feed? Odd that the feed store didn't notify you they were going to switch out your feed, assuming what you got is similar to what you were feeding. I've found when switching feed, it's best to do it gradually. Prices have gone way up on the feed I use but rather pay the price that have issues which I have had at times in the past.
Can you take a picture of the poop? IF this started about the time the feed was switched, I'd say that's a possibility. Have you checked the feed that it smells & looks fresh? Is it possible your girls picked up some decaying tidbits while out and about?