Strange. It seems the particle had its polarity inverted very quickly.
by Whoandwhatitis moderator
If you check Slice view, you can see that this is a gigantic oval. In the Normal and Zoom views, this appears as a much smaller, very elongated oval instead.
This is normal, and it's only generated by the computer:
The FAQ briefly mentions these circle/oval shapes:
Q: What are those colourful circles or dots I see sometimes? - A: They
are artifacts from the computer. Ignore them.