Yep, it's bacteria and crap that has grown in your air con system. If you pop along to your local car accessory
shop you will be able to buy stuff to clean the system.
I know comma make a product that comes in a can. You simply turn the air con on and hit the recirculation button on the dash. Open the can and let it do its stuff , but make sure you are not in the car or your lungs will get the same treatment
. Hay presto..job done.
I have yet to try it but I’ve seen a few posts about this stuff and it’s supposed to be good.
Hope this helps.