First year with pool. Pool builder winterized my pool this fall. I have a pool spa combo. He closed/winterized the pool and set it so that we could still use the spa during the winter. I just went outside this morning to put the pump cover on the pool and noticed that my entire pool is frozen over. I have an automatic cover on the pool and i can tell that the cover is frozen solid to the frozen pool water below. Is my pool supposed to freeze over in the winter?