Leaking, from breast. not pregnant?

my sister broke her leg because of the icy conditions outside. she has a newborn baby and i've been taking care of it. about 24/7, to help her. today my breast were super itchy. i pinched my nipple don't ask me why, but what looks like milk leaked out! what is going on? my sister finds this hilarious. i don't. what's wrong with me? i'm freaking out i'm 19, in perfect health.