High impedance divider measurement with instrumentation amp
You have three DC voltage points: plus 5V supply, its counterpart which is drawn as ground symbol and the signal GND in the middle of +5V and its ground.How many volts is the voltage between Vinminus and the signal GND. If you cannot guarantee it's less than 2,5V you are out of the proper operating range of your components.

BTW. Your divider seems to be 1000:1 and you wrote 100:1