What can cause fireplace to all of a sudden start letting smoke inside?

it has been working just fine since late Oct. all of a sudden last night it stated letting smoke inside the house. i checked it and its not clogged, smoke is also coming out the top, and its not that dirty on the inside of the chimney. i have already scheduled a chimney cleaning, but since it doesn't look that dirty im worried it wont fix the problem. any ideas?