I have a differential drive robot and I need to get the angle of the drive wheels. The wheels are turning about the y axis.

When I get the links of the wheels and use

wheel_link->GetRelativePose().rot.GetPitch();

The angle values goes from 0 to pi/2 then back to 0 and then -pi/2 like shown in the picture below

How do I get the correct angle of the wheel? Why does the quaternion not get converted correctly?