I'm sorry for your troubles! It sounds like your ovaries have reverted back to reproductive mode. (Chemo and radiation induced menopause can be temporary.) It's pretty typical to have irregular bleeding during perimenopause so this may be all it is. But even after menopause, women can have bleeding episodes since the ovaries never completely shut down. In most cases, it's just sporadic ovarian activity but of course needs to be investigated to rule out anything serious.
Of course some breast cancer treatments can cause uterine problems such as polyps and even endometrial cancer, both of which can cause abnormal bleeding. Hopefully, this proves to be nothing serious just like your previous episode!