@MarqFJA87
3 things.
1) Not using a nerve makes it more sensitive to future touch. Since she never used them, they likely were very sensitive.
2) The location is not entirely not erogenous, it is also in proximity to even more sensitive areas.
3) She had never felt anything once becoming an undead, now she can, the sudden influx in sensation information likely is very overwhelming.
Bonus: Fan-service because Japan demands it.