Your remote control to the car alarm (aka key fob) is not working reliably. You need to get into your car or at least disarm the the alarm on-demand. How do you get the keyless entry device (or the alarm remote control or key fob) to work reliably to it will deactivate or activate the alarm when you need it to happen?
Possible solution #1
Tighten the screws in the back or the side. If the remote control (or key fob) has an intermittent problem, tighten the screws as much as possible. Loose screws may create the symptom of an intermittent problem with the remote control of the key device. You may want to buy a Leatherman device with tiny eye-glass Phillips screwdrivers. Loose remote control casings can make the buttons not work every time.
Possible solution #2
Replace the battery in the remote control aka keyless entry device or key fob,