Six months or so ago, I heard the phrase "come over all unnecessary" for the first time and was unable to determine what it meant. The SERPs have improved slightly and I see now it means "to become sexually excited." You can "come over all" other things too, as in this passage from Winnie the Pooh by A.A. Milne: "So with these words he unhooked it, and carried it back to Eeyore; and when Christopher Robin had nailed it on its right place again, Eeyore frisked about the forest, waving his tail so happily that Winnie-the-Pooh came over all funny, and had to hurry home for a little snack of something to sustain him." This example seems like a case of illogical causation but I guess Winnie the Pooh's response to all kinds of situations and stimuli was to eat something.
Anyway, we can gather that "come over all X" means to be suddenly overcome with a feeling or emotion. (According to Mary Ruefle, "Neurobiologists have distinguished emotions from feelings.") I find it quite peculiar that "unnecessary" should mean aroused, but maybe this expression is only used in situations where the stricken can't do anything with their arousal, because, say, they're in the middle of a multi-course dinner party.
This reminds me of a guy I met a year or so ago, who told me that he had always thought the word "undertaker" was very dark, as though it referred to the literal process of taking bodies down, under the living and into the earth. It had always had those grim connotations for me too. But he realized, he told me, that in fact it was just "the ultimate euphemism" -- an undertaker is engaged in an "undertaking."