Is it possible to gain 10 lbs of just muscle?

I am 16 years old and have been running for about three years. I was doing around 2-4 miles every other day until last year when I began running 5-10 miles every couple days and spinning 3-5 times a week. I am 5'6" and used to weigh 111 lbs. My BFP was 11.43%. In the year since I began running farther I have gained over 10 pounds and my BFP is now 16.32%. I have always been an extremely healthy eater and that has not changed. In Fact when I look in the mirror I feel I am thinner. People ask me if I have lost weight and when I tell them I have gained 10 lbs they are shocked. The only place I really do see a difference is in my quads, they both look as though they have doubled in size. I am pretty sure it is all muscle but sometimes the scale number and 5% fat gain freaks me out and I feel like I need to lose it fast. I just wanted some other opinions.