All Of A Sudden Today, A Circular Dark Purplish Bruise

All of a sudden today, a circular dark purplish bruise appeared on my inner thigh about 4' down from my inner leg. It is the size of a quarter and nothing happened to me to induce such a bruise. It seemed to spontaneously appear. Any idea what it is? Should I be concerned? Should I see a doctor? It doesn't itch, it's not raised or swollen.

Thank you

K