What Causes Excessive Sweating From Anal Area?

I'm 14 and I am worried about excessive sweating from my anal area. I am just over 6 foot and weigh about 13 1/2 stone? It leaves wet marks on seats at school and it is extremely embarrassing and makes me anxious to hide it. I only really sweat a lot from there and my armpits, even if it isn't warm. Help??