Losing weight is never 'easy'. Losing weight is simple mathmatics. At least it sounds easy on paper ;)

losing weight is calories in/calories out - whether you are on birth control or not. Life can cause weight gain. College (that famous freshman 15!), marriage, eatin gout, etc. All things we do as a matter of life. People are very seldom the weight they were in high school. A lot of people try for that weight though! ;)

there are weight calculators on the internet that can tell you what your calorie count should be if you are trying to lose a pound or two a week.

Good luck!

Heck, some people can eat all day and never gain, others just look at food and gain it also depends on metabolism and some with exercise.
A friend of mine's daughter had the same problem with her b/c pill, her dr recommended a iud and it really helped her of course we are all different but you might ask your dr about it.
Good luck!