Why won't my weight stabilize?

I went away to camp weighing about 118 pounds and when I came back, I was at 113. That was two weeks ago- since then, i weigh myself every couple days and my weight is always drastically different than the last time I weighed myself! Granted I haven't been excercizing lately but, this is just irritating- I can go from 113 to 118 in about 2 days, and then back to 116 a day later. why is this happening, what should I do to make it stabilize, and what would be an ideal weight for me (at 15 yrs old and 5'2) THANKS!