You should always start off with getting it scanned, autozone does free scans and this could lead you in the right dir. Try looking under the car to see if you can notice any part of the exhaust broken or missing, also you may want to start it up and see if you notice any noises in the exhaust spec. around the cat. or cat. converter. Also would be interested to know if there is any other change in the performance of the vehicle. Thanks for the question.
If this was helpful to you please select accept, and if you have any other questions please do reply. Thank you Eric