My subaru keeps stalling

Hi, I just got my clutch changed, and everything was rolling just fine for a few days, until out of nowhere, my car just started stalling at stops, or when the car would almost be at a stop. It idles very strangely as well, it gets really low when I slow down, My mechanic can not find what is wrong with it. I need to find out what the problem is before it becomes a huge expense to fix. PLEASE HELP ME!!!!!!