A tubal ligation in and of itself won't cause changes in weight. But you said your periods are very irregular. That can be a sign of some sort of hormonal problem such a thyroid trouble, estrogen/progesterone balance, polycystic ovary disease and other things. Those can cause weight gain or weight loss, depending on the problem.
Another possibility is that as we age, we just can't eat as much as we did before. Our metabolisms worked very efficiently when we were brand new human beings. As our bodies get older, these systems work less effectively.
A trip to the doctor would be in order so see just exactly why you're gaining and having irregular periods. If it's a hormone problem of some sort it may be fairly easily fixed.
Good luck to you.
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Weight gain after tubual ligation. 
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