In a tubal ligation how come the egg cant be fertilized but you still have your period?

This is because the egg is only physically barred from getting through. The uterine lining still thickens and builds up during the cycle as no-one told it there was no point! Your hormones still function in the same way, producing and releasing the egg, and building the womb's lining. The period still has the function of cleaning out this thickened lining each month.