Erlend Langsrud:
From the size of it i guess it is a defocued speck of dust on the flattener lens. (The one you see when the camera is removed). Looks odd though, but that could be because it is not quite succesfully removed by the flatframe.
I agree with Erlend. I got a similarly size spot on my RASA 8 recently. Turned out mine was dust on the Celestron clear filter. I suggest you retake your flats and see if you can see it in your flats.
Good Luck!
Ted