On drum brakes, ocassionally something as simple as a small pebble can get in there and create a squeek. Your best bet is to inspect the drum/shoes for an obstruction. Otherwise, take it to someplace like Midas who will(most likely) give a free assessment of the problem.
My advice always is, if you think there is something not kosher with your brakes, have them looked at. They may save your life one day.
