Does flexing during a blood pressure check increase blood pressure? i think either my arm is too big or my cuff is small.

Blood pressure : All the above may alter a BP reading You need to keep your arm still and make sure your cuff is the right size

Yes: There are many technical issues that can raise blood pressure readings. Most cuffs have markers to indicate if they are appropriately sized; you can check this. Any movement can increase blood pressure. It is optimal to be at rest for several minutes before checking BP, and to have minimal emotional or other stress for most accurate result.