Probably the drain for your evaporator has clogged up. The evaporator is located in the passeger footwell area on most cars. Underneath the car there is a drain that lets water that forms under condensation while using the ac. Ever notice the water dripping under a parked car. That is water from the evap.
Sounds like yours is clogged and growing mold inside of the evap box. Its a easy repair that you can do or have a good ac shop do. There are ac cleaning kits out there (like BG products or Liqu Moly) that sterilize the evap box and you fix the clogged drip line. Then you are good to go.
here is a link to basic ac sysytems...
http://www.familycar.com/ac1.htm
Dackel