A couple things tho -- its not all bodyfat that you gained -if you did a radical change in diet & started eating crap, the good news is that you are probably holdign all sorts of water, and also you should go back to "normal" very quickly because your body hasn't really had a lot of time to settle into a new "set point" as far as adjusting its metabolism around your "lifestyle".
Just so you can have a solid grip on what you are eating, I'd suggest putting your speicfic meal plan into a food counts program like
www.fitday.com -it will tell you exactly the total calories as well as the macronutrient breakdown - %, grams of proteins, fats & carbs. The point is you want to eat enough calories so your metabolism doesnt' slow down & you can establish an efficient burn rate, but also make sure you are eating the right ratios of stuff to both fuel that burn rate (carbs) as well as ensure you arent' eating away your muslce mass --- more bodyfat always looks better when it has muscle along with it isntead of no muscle at all.
Also can you tell us your stats - height, weight, est bodyfat? "I gained 20 lbs in a month!" doesn't really tell me where you are at now. Also that helps us set an optimal total cals & ratios.
Generally in teh diet you have no good quality fats or carbs. Fats help provide energy when you are on a low carb day, but if you stay low carb / no carb all the time, even Atkins people will tell you that you will drop a whole bunch of water weight but you will feel like shit, be moody, have headaches and be tired all the time. THat impacts the quality of your daily life as well as leaves you w/ nothing to run on when it comes time to hit the gym. You can do that for a short period of time but your body will figure it out & start to bog down your metabolism and when you start to eat normal again you will gain it all back.