Cheryl,
What kind of batteries are in the 5n1? Some kinds of batteries affect the thermo/hygro sensor and throw it off I've found in hot weather. I suggest batteries that are name brand, such as Energizer, Duracell, Everready, or Rayovac.
If you have those already installed, take them out and you will see a switch in the compartment with A B C selections. Take note of where the switch is. Next move it back and forth, then put it back where it was, and install the batteries again. This will do a reset of the unit.