Why does my car shake/shudder when stopped and not shift correctly?

I have a 2000 Infiniti G20T, and it started having problems recently. First my check engine light came on a few days ago. Then today I noticed I was sitting at a red light and my car was shuddering moderately. Then, as I'm driving I feel like it's taking longer than normal for my car to accelerate. At one point, I pressed the gas and the car didn't go any faster... I mean it really lagged and then finally started to accelerate. Can someone help me? I think my car is falling apart! Thank you!