2003 Ford focus se
 

I have had a 2003 ford focus for about 4 months now. Ever since i have owned it, the car shakes while idling and while at a stop light, the shaking gets worse if i have the air on while stopped. The car doesnt shake while in motion, also there is no check engine light on, and i just updated spark plugs, air filter, oil. What could be wrong with it?