This is most likely lens flare: one of your lights is shining directly into the lens or catching the front element at an angle. Dark clothing makes the flare easier to notice.

To fix it:

• Make sure no flash or modeling light is aimed toward the camera lens, even slightly.
• Reposition the umbrella/softbox so their light is only hitting the subject, not the lens.
• Use a lens hood to block stray light.
• Add a flag or mask between the light and camera to shield the lens from spill.
• Check for any bright light sources just outside the frame.

One answer also noted a separate dark spot that could be sensor dust or something on the background, but the bright circle itself sounds like flare.