During pregnancy, BP should be measured in a sitting position with adequate back support, feet flat on the ground, empty bladder, and no Tea/Coffee in the prior 30 minutes. BP cuff should be at the level of the heart.
Source - ESH2023
Next Steps
Consult a General Physician for in-office measurement of BP with a mercury apparatus and stethoscope for an accurate reading.
Health Tips
The difference could be because of user error. That is why you should take 3 readings and take average of the last 2 readings and note it down.