Mild to moderate discomfort is sometimes reported after a root canal procedure. It's rare, but unfortunately it seems u have it!
As suggested by the earlier doctor, make sure you take your prescribed medication. The rule of thumb is that the pain/ discomfort should be reducing. If it is not reducing, then you need to go back to the Endodontist and resolve the issue.
Next Steps
Inform the treating doctor that you are experiencing this pain. Keeping him/ her in the loop always helps.
Health Tips
Do not panic, and also do not unnecessarily suffer. Dental treatments, including root canals are nowadays almost always painless.