I am having a nose job in December, I have hit my goal weight and maintained it for the last 5 months. Will I gain it back?

Many of my patients claim to lose weight after rhinoplasty, as you may temporarily lose the ability to taste, your upper lip can be stiff, and your diet may be limited to liquids and soft foods.

After your surgery, focusing on your recovery and following your post-operation guidelines is very important. Along with these instructions, following a healthy diet is helpful for weight concerns. You may wait a few weeks to weigh yourself, for your healthy recovery is what matters most. Once you are feeling better and swelling, bruising and other postoperative effects are improving, you can weigh yourself.

I don't believe nose jobs and weight will relate very much.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search. 

There is a lot that happens to your physiology following surgery.  Stress hormones are released which cause a bit of temporary fluid retention.  You can't exercise for some time as well.  I would suggest not looking at your weight for a few weeks after surgery and concentrate on eating healthily.  Best wishes.

After a rhinoplasty or any surgery, rest is encouraged to reduce swelling.
Weight gain is primarily controlled by limiting caloric intake, which is minimized by eating mostly fruits and vegetables.
Most exercises allow you to burn about 500 calories an hour. So if you miss your daily hour at the gym , you may not burn  7x500=3500calories, and gain one pound (=3,500 calories) from not exercising. 
This is reversed with dietary discretion or cutting 500 calories in your typical diet
daily .
Joseph Pober, MD, FACS
Harvard & MIT trained plastic surgeron

Concentrate on eating healthy and take nice peaceful walks.  This will help with recovery as well as your overall health.