Although shouldn't is derived from should not, the two are not grammatically interchangeable. While "Shouldn't I do it?" is grammatical, "Should not I do it?" is no longer acceptable. It would instead be "Should I not do it?"
1983, Zwicky, Arnold M.; Geofrey K. Pullum, “Cliticization vs. Inlection: English n't”, in Language, volume 59, number 3, pages pp. 502–513: