It looks like a reaction between the paint and primer. The low temp may have been the culprit as the low temp let the solvents stay in the paint longer giving more time for a reaction to occur. You have nothing to lose by sanding and reshooting that spot, so give it a whirl. FWIW I never go finer than 400 grit before paint. IMO the 400 is fine enough and gives a better bond between paint and primer.

Ken