You have encountered the situation that most commonly generates freak waves. Probably the deadliest version of which is found off the SE coast of Africa, where the Cape rollers meet the Sw'ly flow of the Agulhas current. Many a large ship, and I do mean large, has had her back broken by encountering a freak wave there. A freak wave is generally considered to be one that is impossible to generate by wind. In fact, their occurence was denied as impossible for many years. We now know differently.
One of the conditions needed for these to form is shallow water. "Shallow" in this case may mean anything under the 100 fathom curve. Shallow water waves, so familiar to the Great Lakes sailors, have a very short wavelength or period, and will build up very quickly to a surprisingly large height. The lack of depth of water, compared to open ocean, is what causes this phenomena. You, more commonly, observe it at the beach as the waves approach the shallows building in height as they do so.
Now, if you take a wind opposed current and add it to the mix, the effect is exacerbated. It makes for a most unpleasant ride, does it not? And, a big part of the problem, is that it is the type of condition that can blow up in very short order. Maybe twenty minutes of wind is sufficient to significantly change things.
The sensation you are probably feeling on your rudder is the result of the short, tall chop produced by these conditions. Rather than the boat riding up and down a swell the seas are passing quickly under the boat, and the rudder is being exposed as the trough passes astern. It has a sort of a weightlessness feeling to it does it not? That is because it is either out the water or the water is what might be described as thin or less dense. The rudder is less effective, must be held over for a bit longer until it's sporadic bite grabs water.
Once out in deeper water, the condition moderates. The danger, on a boat, is that conditions can be quite benign and you can go forward, only to be slapped right in the chest by a green sea six feet above the waterline. All while the boat is not necessarily pitching that much. Quite disconcerting, yes?
Bowditch has a chapter on ocean currents that is quite illustrative of their nature and interaction/forming related to wind. It is a good primer on the interaction of wind/water and moving masses of water. I am not aware, or do not recall, specific names for these conditions. A particularly spectacular, and dangerous, example of this occurs at the river Columbia bar. It is not for nothing that the USCG chooses to train their motor-lifeboat operations there. You've probably seen the videos of their forty footers rolling over 360 degrees. The videos were shot off Cape Disappointment.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.