What is that SMELL?!

OK so this is day 2 where I have been using fleece in my boys cage. I was VERY nervous because I really didn't think that it was going to work! but, I gotta tell you, it's so much easier then the Carefresh I was using....anywho, back to my question....the first night that I used fleece, everything seemed to be going fine. Then when I woke up this morning, there is a very nasty smell coming from inside the cage. I stuck my head inside and I cant figure out the smell! I fed them pickles with their normal dinner and I thought that maybe it was the pickles. But it wasnt! The smell is still there, but it's not as strong. They seem to be liking the fleece since they're all sprawled out on it but the smell is driving me crazy! Anyone have any ideas to what it could be? Is it the fleece? I've smelled everything in the cage and can't figure it out! Maybe it was the pickles...lol