It means that your budget isn't in good shape because of your overspending so your budget will be tight and it implies that you may need to cut back on your spending for some amount of time. It might take you a few months to get your budget back on track.
Because you spent too much money for Christmas, you will have to be careful of your budget. Your budget is how much money you are allowed to spend. So because of all that money spent, your budget will be higher the whole year.