why am I gaining weight?

Question
first off, i am a 20 year old female of about 170cm and unfortunately now 70kg.

about four years ago now i lost approx 40kg going from 93kg to 53 kg.

for about 3 years i maintained a weight of approx 55kg until mid last year when i started gaining weight without changing anything.
i exercised everyday (brisk walking for approx 1 hour) ate a low carb vegetarian diet.
My weight is now up to 70kg. I have been seeing a doctor but she cannot find any reason for such a large weight gain. And after first seeing her when my weight was at 65kg it went up 5kg in one month between visits. After the shock of 70kg i got my weight down to 66kg again but it has again gone up to 70kg. I have increased my exercise from walking to jogging at least 45 minutes/per day with 15 minutes walking and in the last 4 weeks have started swimming 3 times a week on top of that.
I cannot understand why I can't lose weight, when friends from school are losing kilos per week with more food and less exercise.
Do you have any suggestions which might assist my struggles?
At this stage I would just like to get back down to 60kg however I have been my happiest and most confident at 55kg so that is my goal.
Thank you and look forward to your response.

Kate

Answer
Dear Kate,

My first thought is, "what can you eat on a low-carb vegetarian diet"?  It sounds like all you may have been eating is vegetables!  If you were eating 800 calories a day, which is extremely low, that may have been keeping your weight down to what is quite low for your height (according to the ideal weight tables, and my professional judgement) regardless of how comfortable you were with it.  It may also have slowed your metabolism over the years so that now when you try to eat a normal amount of calories (like 2000) it causes weight gain.  Going off a low carb diet also causes immediate weight gain in the form of fluid (because on a low carb diet a large part of what you lose is just water weight).  
The weight gain, if caused by a lower metabolism, will tend to level out over time, as your body gets used to being fed again.

How did you lose the 4 kg a few months ago?  That is a LOT of weight to lose in one month and another thought is that the weight you want to lose may come off slowly, but you may feel frustrated with 2 kg per month and feel like nothing is happening.  Keep a weight chart and log what you weigh every week or so to allow yourself to be objective about what you are really losing.

My best advice when people start gaining weight with no apparent change in diet is to start keeping a food diary.  Often we have started a new habit that we aren't consciously acknowledging... maybe a new snack or a treat at a friend's house, or picking at food while we cook... writing down everything you eat will make it very clear exactly what you are eating and you may find the answer there.

Otherwise, I'm glad you have seen a doctor and she has apparently ruled out thyroid problems (which are common at your age, easily treated, and earmarked by causing weight gain and fatigue for no apparent reason).  If you haven't been able to solve the problem in another month, consider making an appointment with a registered dietitian in your neighborhood so she can do a personal and thorough evaluation of your eating and exercise program.

Good luck :)

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