At 14, your body is building itself up to get ready for having babies, and maturing into the gorgeous young woman you will become. It is perfectly normal for many girls to put on an extra 5-10 pounds around 13-16 as their bodies get the whole puberty thing sorted out. Sometimes this looks like a bit of pudge, especially around your hips, but it's going to go away on it's own so don't worry about it too much. That said, at your current weight, you are on the low end of normal according to
http://www.halls.md/body-mass-index/bmi.htm (a bmi calculator that takes your age into account).
Learning healthy eating habits will serve you well as you grow older, but having a piece of cake once in a while won't hurt you. With my girls, we have a house rule that they are allowed two 100 calorie treats per day. A treat is anything that has virtually no nutritional value except carbs or fat, or is extremely processed (including sugary drinks!). They also get one 'free' day a month, usually for a birthday party or special occasion, and are fairly active children. I suspect that if you come up with similar rules for yourself, eat when you are hungry, and try to focus on eating nutritionally dense foods (fruit, veggies, lean meats, beans, seeds & nuts), you will find you maintain your already healthy weight and enjoy the benefits of good nutrition.
Have you talked to your mum or your doctor about food choices? I would strongly suggest that you go and talk to your doctor about your weight and staying healthy if you are at all concerned about it. There are lots of important growing things going on in your body right now, and a lack of the right nutrients can really throw a wrench in the works. Your doctor will be able to tell you more about what a healthy eating plan will look like for you so you don't miss anything important, and what a healthy weight range is for you. In the meantime, give yourself a break - a couple sweets aren't the end of the world.
So Today I fell off the wagon, hard. How do I get back on?
My mum pressing me to eat some was just enough to push me over the edge. I don't even want to think about how much cake I ate plus I felt so guilty that I decided to go and eat a kitkat (I know, no logic whatsoever in that). I feel truly awful now and I know I've gone way over my target calorie intake for today so I guess I'll just have to try and make room for extra exercise in between revising for even more exams this week. I'm in need of some serious motivation! 
