10 pound in 10 week random gain? I don't get it.

I'm 5'9 or 5'10, 19 year old female. I lost about 30 pounds and maintained at 140 for almost a year. All of a sudden, in the last ten weeks or so, I have put on 10 pounds.

The only differences in diet are that I have been eating more carbs but I have also been exercising more (It still equalled about 1600-1800 calories a day). I swam almost 5 or 6 miles a week for 8 of those weeks. I'm living in Spain right now so there is more oil in the food, but I have not consumed enough extra calories to gain all that weight.

Any ideas why it came on so fast?

What can I do to take it off again?

Ok, since I, my older daugher, and my older sister all experienced a similar thing, I have some ideas.  There are many people on the Motivational forum that know more about this sort of thing; but I have done quite a bit of research on this--and it really helped me lose weight and yet still eat a healthy about of calories:  1) it is my understanding that the body, when someone starts cutting back on amount of caloires consumed--particularly cutting back too fast--changes its basal metabolism needs.  That is, say in my case that my bodys BMR was 2400 calories needed, and I jumped down to 2000 calories taken in, plus, I exercised an additional 500 more calories off.  Now, I have dropped down to only taking in a plus gain of 1500 calories.  What the body will do is then slow down its metabolic engine and, suddenly, I would find myself only needing, for example, 1800 calories to maintain the basal metabolism.  So even when I cut back on calories and exercised more, the weight quit coming off because the body decided that I didn't need as many calories to run my metabolism.

Here are two things that I did (and I didn't come up with this--I got this from many health sites and health professionals and from people who have done these things):  A) make certain that you have a very healthy, high protein breakfast.  First, drink a large glass of water.  Recent studies suggest that starting the day with a large glass of water will help kick-in your metabolic engine.  Second, eat around 45-60% of your daily protein needs in that first meal of the day.  There are many foods you can check on this site to find ones high in protein and that are healthy.  The body will use this protein input to really jumpstart your metabolic engine.  B) Eat around 5-7 smaller meals in which you keep your targeted calorie intake.  Write down your calories for each day until you know you are being consistent.  Include, assuming you are not allergic to some of these things, such items as walnuts or almonds (1/4th cup daily), beans, potatoes, fruits, vegetables, whole wheat pasta/flour/bread (assuming you are not allergic to it), nonfat cheese, egg whites, etc,, etc. in your daily meals.  You might also consider taking flax seeds or fish oil.  Yes, watch the sugar, the transfat, cholesterol, and saturated fat, but generally a lot of fiber, fruit, and vegetables will negate if you take in a bit of those things.

Conclusion:  I, my sister, my nephew (who runs a health-wellness clinci), and my daughter have all lost weight and kept it off so far following the above guide lines.  And, yes, we all have included exercising  as part of that.