losing speed,and shift knob problem

I just bought a new exhaust and i put it in my honda accord lx 1993,now i was driving the road goin to school.What happen is,while i was driving i push the gas all the way down and it was goin slow,it wasn't like that before and my exhaust sound is goin down.When i was putting the shift knob to drive 3 the green light came on,but the light on drive 4 is still shining.And is still the damm thing is goin to slow i don't know what the hell is wrong with this.Somebody please tell me.