It Hurts To Stretched Out My Arm. Like Burning Or

It hurts to stretched out my arm. Like burning or stinging in my left bicep on the inside of my arm. Like something is pulling my skin. Doesn't hurt unless I stretch or touch it. Started yesterday and woke up today with the lower part of the same arm hurting the same way. Hurts when I bend my wrist but the wrist itself doesn't hurt. Now there is a bruise on my bicep trailing down just like the pain. No injury at all. No exercise. What causes this?