thank you for your query.
after tubal ligation
is rare, but not uncommon.
What is more worrying is that, in case you conceive after tubal ligation, chances of an ectopic pregnancy are high, that is pregnancy outside the uterus.
Your cause of erratic bleeding could be a pregnancy, or an ectopic pregnancy.
Rarer causes are age related hormonal
changes, stress, anxiety, weight fluctuations, emotional trauma, thyroid disorders, polycystic ovaries
you need to get yourself checked by a gynecologist at the earliest.