I changed my alternator and battery on my car. It ran perfectly for a week. After exactly a week my battery light is coming on again.

Hello, "new" alternators can be faulty alternators. Depends on quality of the build. Definitely check to ensure the drive belt is still engaged and not detached or loose. Other than that, charging system testing is necessary. Note that the wiring connector at the alternator should also be carefully checked for damaged or burnt terminals.