What is coming out of my cats butthole/vag?

Okay, my female cat is 14. I adopted her from the humane society and one of the stipulations to taking her and her sister home were they had to be spade. (6 wks) I know they don't do it that soon anymore. Anyway, her whole life she has this brown liquid squirt from her backside. It smells very strong, and male cats go insane around it. The smell is raunchy, it makes my husband gag. I'm thinking its some kind of pheromone?? It doesn't happen often, and has never negatively impacted her that I know of. What is this stuff?