Since you had a surgery on your spine, it is advisable to avoid pregnancy
for at least 6-12 months as pregnancy puts pressure on the spine which can be difficult in your case. You should exclude/confirm pregnancy through a trans-vaginal sonogram and plan further management accordingly. If negative, hormone treatment given will induce a withdrawal bleeding 3-7 days after completion of the treatment. Most often, whenever the body faces some trauma, it is common for a delay to happen in the menstrual cycle. Wish you good health.