Does anybody know what this is on my underwear ? should i be worried ?

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.

It's probably just dye cuz it's white and was washed with colored clothes in the washing machine

The answer would be a true statement