I Have Tiny Black Dots The Size Of Black Heads

I have tiny black dots the size of black heads on my wrist. They are all in a circle, and found no where else on my body. I have lots of freckles but these are much darker and are easy to tell apart, they are not regular and they seemed to have appeared last week and gotten darker. I originally thought it was stubborn dirt but it wouldn't come off in the shower. Should I be worried? Wjat could this be?