What’s happening?

My 3 month old has hardly been pooping the past few days and the few dirty diapers he does have are super runny and smelly. I’m taking him to the pediatrician tomorrow but does anyone know what could cause this or how to help him in the meantime? He seems to be acting fine otherwise, but he’s a little clammy on his hands and feet and he suddenly has a skin reaction to Aveeno products