Hello and welcome to JustAnswer!
What model is your vehicle?
Is it the factory alarm that is going off?
yes it is the factory alarm
The year of your vehicle did come through in your first post, but not the model. What model is your vehicle?
On the back of the lock cylinder in each door that has one there is a microswitch. This switch is wired to the body control module and tells it to disarm the factory security when you have entered through the door.
If the alarm no longer disarms when you unlock the driver's door with the key then the lock cylinder switch in that door is likely the problem. Replacing it would involve removing the door panel and removing the switch from the back of the lock cylinder and replacing it.