It is possible that over the time you spent shooting your Hill, you may have changed your grip or alignment. I think most people take a firmer grip on their longbows than they do on their recurves, and possibly bend their bow arm elbows a little more. When you went back to your Lee, possibly you didn't use exactly the same grip you did before. A change in the pressure point of your hand against the bow grip can change the point of impact of the arrow.
The important thing is to be consistent. If you knew you were going to shoot 6" high from now on, it probably wouldn't bother you very much. You could just adjust to it, or you could raise your nock point 1/8" or so and the problem would go away. Your problem is that you don't know why it happened, and so you don't know how to keep it from changing one way or the other.
Your point of impact is controlled by several things, which you probably already know: arrow weight, draw length, anchor point, how you aim, and your bow grip. If you keep all those constant, your point of impact should stay constant. You must be changing something in one of those factors if your point of impact is changing.
For example, if I shift my focus from the target to the arrow point, I get a high flyer. A change in the relative finger pressure on the string among your three fingers can change the point of impact. You mentioned your shoulder; if your bow arm shoulder is low one shot, where it should be, and high on the next shot, that can change your point of impact.