When compared to the datasheet the offset voltage which you are getting is very high. Better check for the noises in the ground and try to eliminate it. Use a stable DC voltage to power the instrumentation amplifier.Noise on the supply pins can adversely affect performance.
Place a 0.1 μF capacitor as close as possible to each supply pin. Because the length of the bypass capacitor leads is critical at high frequency, surface-mount capacitors are recommended. Parasitic inductance in the bypass ground trace works against the low impedance created by the bypass capacitor. a 10 μF capacitor can be used farther away from the device. For larger value capacitors, which are intended to be effective at lower frequencies, the current return path distance is less critical. In most cases, the 10 μF capacitor can be shared by other precision integrated circuits.