I have a question about my birthmark.. please help it's annoying me.?

K I have like a bigggg and long red birthmark on my right arm and it kinda goes up the whole arm, but it's not like bright red, it's almost almost the color of my regular skin, but few minutes ago, i just noticed some of it appeared on my neck, and that has never happened... it's freaking me out, like... what if it goes on my face or something!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!…