If you are sure you are not pregnant and have never missed your cycles in the past, then the most likely possibility is the effect of the disease process and the multiple medications that you are currently using as part of the treatment. Th hormone imbalance
created by medication and stress
invariably leads to abnormal menstrual pattern. You may see a gynecologist to exclude involvement of pelvic organs and also exclude pregnancy
through a clinical exam and a sonogram of the pelvis. Hope you find this information useful. Take care.