When a trial court enters an order for alimony, in conjunction with the entry of a divorce decree, under Pennsylvania's Divorce Code, the alimony order is subject to being modified, suspended, terminated, or reinstated upon changed circumstances of either party of a substantial and continuing nature. Such an order, modifying, terminating or suspending an alimony order under the Divorce Code, "shall apply only to payments accruing subsequent to the petition for the requested relief."