Hi Brittany. The air conditioning system recirculates the air within the car or pulls from outside air. With today's cars that air goes through a cabin filter located in one of three places.
I know of one off my head and is the most common location. It is located behind the glove box and usually simple to get to. (Remove the inside of the box, with either screws and more common snaps into place)
If that filter is dirty or full of junk it will cause the flowing air to stink - plain and simple. So, check and replace as necessary. I suggest if you go to a shop request to see it and satisfy your self it is dirty. The majority of shop owner/manager/technicians are trustworthy. Follow your best instincts.
Also, there is an old trick you may try. Or, a couple. First, the recirculated air comes from at the floorboard area, usually the center. Second, the outside air is drawn through vents located near or at the windshield.
Grab your favorite anti-bacterial, smell good spray or pump (environmental friendly is preferred) and have some fun. Fire up the car, set the A/C system to outside ventilation and spray along the vents at the windshield. Let it run 2 - 5 minutes or there abouts, spraying occasionally and sniff :)
Next, do the same procedure for inside recirculation, but spray at the base under the dash near the floorboard. (No promises, but usually the center is where that intake duct is). Again, let it run a bit, spray intermittently, and sniff. :)
A good thing to do is jump outside for some fresh air sniffing then go back in to compare. No promises, but that is what I do. Follow that up with a scenic cruise along a Hawaiian beach with all the windows down and the vents off. Then re-test.
That leads into sniffing and scurrying instead of hemming and hawing. Or, get the book 'Who Moved My Cheese?' for a good read.
(P.S. My mom was an Occidental graduate. By proxy I'm acting in her behalf assisting a fellow alumnus :)