This photo show what has been called "the red spot issue"; a reflection of the light exiting the lens's internal iris diaphragm, and that "exiting" light reflecting off of the FRONT of the sensor or the sensor's anti-aliasing filter array. This issue is fairly common on this lens, the 50mm f/1.8 AF Nikkor, as well as on other lenses when used in this kind of backlighted scenario. I posted this to this thread, in which a poster asked about the issue with the 50mm f/1.8 AF-D Nikkor lens. Since I had experienced the issue just a few days prior, I posted this photo as an example!

http://www.thephotoforum.com/forum/nikon-lenses/359782-purple-spot-50mm-f-1-8-a.html