I have a 140 year old Victorian home I've restored with my wife the past 6 years. One of the things we did in the exterior was to remove some very old and corroded tin from the back of the home that was wrapped around our brick exterior. In the past few months I've noticed we are starting to see a mold problem on that very wall interior which is our bathroom. Is it

A. a problem with steam ventilation inside or

B. a problem with moisture finding it's way in from the exterior.

A note, there is one section of the interior bathroom plaster on that wall that has "puffed" out in two spots so I'm not really sure what to do.

I don't know if I need to seal of the exterior brick which is painted already or fix the inside>