Yes, you are right that you can get pregnant with a tubal ligation
also. This is when there is failed ligation but this would have occurred soon after ligation.
The other possibility can be an ectopic pregnancy
where in pregnancy is outside the uterus
mainly in the tubes. This happen when tubal ligation ring causes damage to the tube promoting fertilization in the tube.
You can get a pregnancy test to rule out these possibilities.
But if its negative, your light periods can be attributed to your age where in ovarian function ceases and you approach menopause