Read 4wire pressure transducer via a differential amplifier
The part number you have, M3021, is a mV output device, so according to the datasheet should have a load resistance of minimum 1 Mohm.
Offset voltages at the LM358 may result in a voltage close to or even below ground. Although the input common mode goes below 0 V, the output certainly doesn't. Your amplifier may not be driving the output correctly near to ground.
The arduino ADC does not read correctly within a few counts of ground.
That's three reasons you may be having problems.
As the LM358 is a dual opamp, I suggest you use the basic two-amplifier instrumentation opamp circuit shown here. I've arranged the resistors in a configuration that makes it clear how the amplifier works, and is hopefully easy to remember. The amplifiers will do whatever they have to to get the voltage across the R1s equal to the differential input voltage. The R1s current flows through R2s, and we take the output voltage across the whole R2+R1 string.



simulate this circuit &ndash; Schematic created using CircuitLab
Now your high impedance device is loaded only by amplifier inputs.
Use the Vref input voltage to place your output voltage well within the clean range of your Arduino's ADC. Maybe have a diode to ground with a small bias current to give 0.7 V. You don't need to know it accurately as you can take it to another ADC input and read it, so you could use a voltage divider between supply and ground to reference it. The output voltage is the difference between Vref and Vout.
Without RG, the resistor values shown give the amplifier a gain of 11, or with equal values (say all of them 10k) the gain would be 2. You can use RG to adjust the gain up if you want, keeping good common mode rejection, without having to change the matched resistors R1 and R2.
This simple amplifier does have limitations. I suggest you read the tutorial I've linked to for the common-mode / Vref / gain tradeoffs that may cause OA1 to saturate. Either that, or check using a DMM or simulator whether OA1's actual output voltage is within range over the full operating range of your transducer.