Yup. low on charge. Especially if it's making a hissing or whistling sound from the inside when you turn it on. Charge the system up with freon and it should go away and your air should be cold again.
It seems as though Subaru has some pretty piss-poor A/C designs. Most Subaru owners I've encountered have had their freon leak out at some point or another too.
NOTE: the 134A is fairly environmentally friendly. Not as bad as some of the older freon that they used to put in cars.