Thats what I think. Your OpAmp circuit is a noise generator and pickup for noise with that very high impedance of 10M.
Why do you use the OpAmps at all? The pressure sensor output can be connected to an analog input of the ATmega2560.
I would recommend a small resistor and capacitor for safety. You already have 680pF at the sensor output. Perhaps a resistor of 1k to the analog input of the ATmega2560, and an other 1nF to ground at the analog input.