A rough idle at low engine RPM is usually caused by a vacuum leak somewhere on the engine.
So check any vacuum hoses for bad hoses or maybe a vacuum hose has come unhooked since if a vacuum leak is occurring the engine will get a lean fuel mixture which causes a misfire.
Other causes of a rough idle at low RPM are low compression, fuel delivery problems, ignition problems and even a faulty spark plug.
Check the vacuum hoses first though because a rough idle at low RPM is almost always a result of a vacuum leak.
One little vacuum hose leaking can cause a lot of engine problems.