This would be a good question to ask your physical therapist. Sometimes increased pain or discomfort in the central low back area is expected. Increased pain down the leg suggests the need for re-evaluation.
It's important to make sure you're doing the exercises correctly. Starting at a slow pace is often the best. Adding more exercises slowly over time can help you avoid re-injury. Most exercise programs start with easier exercises and then go to the harder exercises.
This type of graded approach can help reduce your fear of back pain with movements. It can also help ensure patients continue to do their exercise programs over time.
Exercise is known to help reduce back pain and disability. Don't give up but don't keep going until your therapist rechecks the way you are doing the exercises and your progress over time.