Calculating Cd from the pressure
 

I am doing a simulation using the Vortex in Cell method and the immersed boundary method using a FEM approach along with SOR , I have calculated the Velocity, Vorticity and drag forces successfully.
the photos are at time step =5 sec

The specifications of my simulation are:
Ly = 48.0 xgrid=384
Ly = 16.0 ygrid=128
Re=100 kinmatic viscosity 1/Re
inflow velocity U=1.0
Cylinder diameter D=1.0
The problem that I am facing is whenever I try to calculate the Lift and Drag forces they become 0 in less than 2 seconds of the flow.
I cannot understand why.
does anyone have a suggestion to solve this?