What other mods are on the car? What downpipe did you choose? Describe how you installed it and and problems or difficulties you may have encountered during the install. Did the car run fine right before the install or did you have any unresolved problems? Did you install anything else or do anything else to the car at the same time?
Do you have a yellow check engine light on in your instrument cluster? If so, you need to have a code reader plugged into your OBD 2 port under the dash in front of your left knee. Most auto parts stores can do that for free. Report that number or multiple numbers here.
More information please.
Without more info, I would lean toward a problem with your primary oxygen sensor or its wiring having been damaged during your install.
