I've recently started a menstraul cycle after 2 years of being menopausal. What causes this?

Rule out cancer. Post menopausal women should not bleed. Sometimes this may be due to a thin lining in the uterus, but it can be cancer or precancer. See a dr. For a biopsy sooner than later.
Needs to be evaluate. Post menopausal bleeding could be due to overgrowth of the lining(hyperplasia), polyps, fibroids or cancer. This should not be dismissed and needs to be evaluated by your doctor asap.