I Had A Colostomy This Past January. I Haven't Had

I had a colostomy this past January. I haven't had any stool output in a few days so my doctor told me to take some miralax. I did so earlier today. While feeling around my colostomy bag I found a round, hard white object in my bag with some runny stool. What could it possibly be? And no, it's not part of my bag or anything.