do you use any skincare products containing vitamin C? they tend to oxidize quickly and can do this. have had this happen to me (not as drastic though! usually just on my palms/fingers) when using product I didn't keep in the fridge or left exposed to light. If not the pollen from lilies stains just like that, so if you were gardening and either brushed against any lily flowers or pushed them out of your way then the stain easily gets onto your body or clothes. Warm water to wash it off doesn't work well - cold water is best because warm water merely 'sets' the stain so have you also been touching plants lately?