I believe it's a color dye from pants or a shirt that must have gotten wet and stained your clothes. You should take a shower to get rid of any dyes that are on your body - if you see a rash see the doctors. To get rid of the stain, use bleach and dip the clothing in wet water and let it air dry.
1 in 5 kids are bullied but only about 30% of kids say they witnessed bullying
