>I thought a relation would be created between the the header, itens and next ID table

No, there is no relation. In your NextID table you will have two separate records. One for Items table and one for the Header table to hold the last ID.

E.g. NextID table will have two fields

Table    ID
ITEM     10
HEADER   3

There 3 and 10 are just examples and they should be the last ID numbers in each of your table.