Welcome to JA.
You probably have a rear air bag leaking. To determine which one you will have to let it pump back up then get a spray bottle with soapy water in it. Spray around the bottom of the rubber bladder on the air bag and if you see it bubble out the it is leaking. They start leaking because they dry rot and crack. This is the most common cause of leaks on this car. They also make an coil spring conversion kit for this car that will completely get rid of the air suspension and could save you money in the future.