What could be the cause/possible causes of abnormal vaginal bleeding or periods nonstop for months?

Too many possibilities to guess the cause. At age 22, it could be hormonal imbalance, depending partly on hormonal contraception dose; infection, including STDs like chlamydia or gonorrhea; and cervicitis (inflammation of the cervix) especially if bleeding often follows sex. Tumors are unlikely at your age, but possible. All abnormal vaginal bleeding needs medical attention: see your doctor ASAP.