I guess I understand though that being mortal would weaken them. In the fact that they would feel aging and bodily deterioration, a feeling magnified by the fact that they've never experienced it before. But, as we mortals know, its not all that bad. They could get used to it. Humans and Elves and Beastfolk have. Its like the difference between someone who has been blind their whole life, and someone who gets their eyes gouged out in a torture room.
Unless, of course, we can say that the feeling of aging and deterioration is accelerated, because the dragons are so old to begin with (ala Dorian Gray). Then I completely understand what you're saying. And it makes sense to tie Dragonrend to "If you shake a dragon just so".
I interpreted it more as they got a glimpse of something incomprehensible and it was very uncomfortable. I'd imagine something similar would be what would happen if we understood infinity. We cannot understand infinity. When you look out into space, you see darkness punctuated by pinpoints of light. You can comprehend that somewhere out there is a ball of plasma burning, but it's really hard and almost uncomfortable to understand that what you are looking at isn't limited by anything. You are, in fact looking at 180 degrees of Everything.* Now imagine having that realization forced into your head while you are hunting or, if you aren't the hunting type, at the grocery store.
*Yes you aren't actually looking at half of the Universe when you look up at the night sky because dust and relativity limits things. Be quiet.