Having your "tubes tied" (medically called a bilateral tubal ligation or BTL), is one of the most effective methods of birth control. A BTL is a surgical procedure that cuts and separates a portion of each fallopian tube. It can either be done at the time of Cesarean section, shortly after a vaginal delivery, or laparoscopically at a time remote from childbirth.
In terms of preventing pregnancy, getting your tubes tied is 99.9% effective. But it is not 100% effective. In fact, nothing is.... Except abstinence.
In a small percentage of cases, despite the tubes being severed, they can grow together again during the process of healing and a connection between the two cut ends of the fallopian tube can reform. This can allow an egg and sperm to meet and a pregnancy to develop.
If a pregnancy does occur after a tubal ligation, it is more likely to be an ectopic pregnancy (a pregnancy that implants in the fallopian tube instead of in the uterus). If you have had your tubes tied and you are getting a positive urine pregnancy test at home, it is very important that you see your doctor to first confirm that you are in fact pregnant, and second, to make sure you don't have an ectopic pregnancy.
So, although getting pregnant after having your tubes tied is highly unlikely, it is possible. If you feel like you may be pregnant post-procedure, the easiest thing to do is to take an over-the-counter home pregnancy test. The more likely explanation is that your periods are changing, as many of ours do after childbirth and as we age.
If your period timing is off, it may also be that something else is going on with your health, which could be affecting your body and your menstrual cycle. Thyroid disease, for example, can cause fatigue, mood changes, weight loss or weight gain, and changes in a woman's menstrual cycle. If you aren't pregnant and your symptoms continue, I recommend having a consultation with your OBGYN.