is it unusual to gain 5 pounds in a year? ( I'm 15 if that makes a difference )

No, not if you aren't quite watching your eating habits :) its normal for a small amount of weight like that to fluctuate throughout the year.

I think I gain 5 pounds in water weight sometimes. 5 pounds isn't anything to worry about as long as you are taking care of your body. Plus, you are 15 and most likely still growing (at least I was). 5 pounds could be from growing. I wouldn't worry about it.

Did you increase in height, bust size too? Your clay is not molded yet. Having some weight fluctuations are normal and healthy. Did you weigh yourself right before your menstral cycle? It could be water weight. Drink lots of fluids, Eat lots of veggies, make sure you have your calcium... then forget about it.