There may be more involved in your weight gain than your diet and lack of exercise. Often the medications you use are the culprit. I can vouch for that. I take prednisone for ulcerative colitis and fight a losing battle against weight gain. Blog4Brains says:

.....Portland Orgonian article, they cited a patient, Becki Welch, who gained 100 pounds in six months on an extended course of the steroid prednisone. Prednisone was prescribed for her intestinal inflammation. She reported that every time the doctor started to taper her off he drug, her inflammation returned and she ended up at 398 pounds.

Prednisone does increase the appetite. A friend of mine who is taking the hormone says that she'd eat the paint off the walls if nothing else was available. But prednisone is just one of many. Others listed on Blog4Brains:

In data revealed on Medscape.com, patients on Zyprexa (Olanzapine) gained 8 to 10 pounds, after 10 weeks. And on PsychEducation.org, it is stated that weight gain can be so large on Zyprexa and other drugs that it can cause serious health complications.

I've only mentioned a couple here, but there are lots of them. If you've experienced an unusual weight gain, check with your doctor to see if any of your medications might be to blame.