The answer to 'duct cleaning blowing cold air' is that duct cleaning can sometimes cause cold air to blow from your vents. This is typically due to the ductwork being disrupted or damaged during the cleaning process, which can allow cold air from outside to enter the system.
Some common causes include loose or disconnected ducts, holes or cracks in the ductwork, or the HVAC system's blower fan being turned up too high. In most cases, this issue can be resolved by a professional HVAC technician who can inspect the system, identify the source of the problem, and make the necessary repairs.