Pressure plot not smooth in dynamic mesh simulation
 

Hello, I've run into a problem that has stumped me and wanted to see if anyone else has an idea. I am doing a multiphase VOF model with mesh motion. The domain looks like a syringe with a plunger. I am applying a velocity input to the plunger to generate pressure in the fluid. When I output the average pressure in the fluid it gives a jagged graph, where the pressure holds for a few time steps then jumps up or down. Does anyone have any idea why this is happening? Output of the plunger position and velocity both gives smooth graphs.

My initial thought was somthing to do with the remeshing as the plunger is moving, but it happens even after the movement has stopped. See attached picture for details.