@coco and @CravenLestat My first thought when I saw her was that she had a puddingish appearance (but not quite because my pudding radar would have made me rage quit) ;)
Thanks for the tip @CravenLestat but I can assure you my caches are cleared after every play session which never lasts for more than one sim day. Even if the portrait panel wasn't recognizing my skin that wouldn't explain the slider changes. My SS was made before I used CC so I know her slider positions are nowhere near zero.
@SemaviLady That may well be the case for some players but I don't have Seasons or moon phases. It's also not mood related because my sims are never anything other than Elated. They are NEVER unhappy. Unrealistic I know but it's my game ;) I do know what you are talking about, though, because I have switched households and seen some very sad looking portraits before I made them happy again. This change of appearance does not look like a mood change as the facial features have actually changed.
@Springfairy556 I see my sim in her everyday outfit when she's wearing sleepwear but I am pretty sure when she is wearing her formal outfit her portrait always reflects that. I should take note of that in future.
@igazor You are a champ! I changed her outfit temporarily and when I went back to her original one the thumbnail immediately went back to normal.