What Causes Excess Sweating?

Hi, I'm 18 years old, female, about 125 lbs and I sweat really bad under my arms. I can put on a shirt and within 5 minutes my under arms are covered with sweat. I don't even have to be doing anything, I could just be sitting on the couch or laying in bed and it still happens. This had been happening daily for about a year and a half. Why am I sweating so much?