sometimes what happens is that the bone of the lower jaws resorbs to such a level as the mental foramen
which is the passage way for nerves comes to lie on the top. usually it lies in the middle of the height of the bone.
in such cases the lower denture comes to lie on the foramen and thus causes impingement over the nerve causing intense pain.
ask your dentist
for an opg xray which could help him ascertain the bone level.
if the problem comes out to be what i have described your dentist has to make sure to relieve that area while making her denture