Is my fiancée's discomfort and hesitation regarding marriage, and her occasional chest pain, after one year of engagement, due to Salatul-Istikharah or due to sorcery?

One should not turn to thoughts of sorcery when problems arise. Rather, one should first investigate natural causes. If the fiancée feels uncomfortable, the reasons should be discussed with her family before marriage, and consultation should accompany istikharah. If there are signs of sorcery, then legitimate Islamic methods should be used to remove it. Among the greatest facilitators of marriage are piety, obedience to Allah, and steadfastness. And whoever seeks Allah's guidance in a matter (istikharah) should entrust the matter to Him, for Allah ordains what is good.