Ay, don't worry too much, mucus in poop is quite normal, especially at 3 months, but since baby just started Cherryberry last night and seems uncomfortable, it could be the new formula or just a tummy adjustment lah. A few of us tried switching formulas and our babies had a bit of loose, slimy poop for a few days before it settled, so monitor for another day or two, if baby seems in real pain, has fever, or it doesn't improve, best check with the paediatrician to be safe, ya?