As I have just self discovered, holding in a sneeze can make veins show on your face! I am now left with 2 dark grayish purpleish looking veins next to my mouth. I assume it is a broken capillary or spider vein or something along those lines. I already had visible veins on the bottom sides of my nose for a while now, maybe for the same reason (I always hold in sneezes) or for blowing nose too hard. But I never made the connection until this morning when I saw these new marks on my face and realized it had to be from when I held in a big sneeze last night.
Really sucks, I'm not sure if this is permanent as I'm 26, but I hope this advice saves others from giving themselves pointless marks on their face.