I would still do the vet get the culture it is definitive. Try checking this take a probed thermometer and measure the temperature from 3 inches off the floor and in the exact centre of the enclosure. The air temp being different on one side to the other is odd air should be circulating that fact you have measurements that are shifting likely represents an error somewhere potentially the air is far cooler than you think.

Do you have control over the ambient air temp ? separate from the surface temps?