You must have a Cap.

A great number of airplanes do a nose down tuck when the rudder is deflected. Some are moderate and others really gross. The pitch is caused by a pressure differential across the horizontal when the rudder/fuselage deflects.

The question needs some more information, what kind of airplane is it, what changed between the flights, CG or weight, incidence angles, etc.?

Aero work is somewhat repeatable and cause and effect is in place. There is always a reason why a change occurs. Just look for what is different including your preferences in airplane trim setup as experience levels increase.