2003 Chevy Impala Acceleration

Engine Performance problem
2003 Chevy Impala 6 cyl Automatic

Hello, lately it feels like my car has trouble going up hills. And although the hill isn't too, too steep, when I change from "D" to "3" or "2" I don't notice any change or force. But when I put it to "1" then I do. Last night I had a hard time accelerating onto an onramp. Then once I got to about 60-70 mph it all felt fine. But then I had to go up a steep, steady long uphill. Now I got up, but I think slower than usual and then the "check Engine" light came on. Once over the hill, it all felt ok, but the light stayed on until I got home. This morning it came on again when I started the car, stayed on until after about 5 minutes of driving. It is now off. But what is causing the slow acceleration, especially on hills, and why don't I notice a change when I change the automatic gear to a lower gear?
Thank you for your help.
V123