Hmm..
It's good, not great, keep working at it..
As you yourself said, "super short short",
I'm pretty certain the concept has been done before, sadistic dentists office, he pulls out a chainsaw and murders you because you haven't been brushing/flossing/whatever.
I smiled when I saw the surrounding objects, continued when the dentist pulled out the magnifying glass, expecting a gruesome death when I saw the cavity, I full out laugh at the chainsaw, but only for a second because it's sorta ruined by the.. geyser-like motion of the blood? I feel you could've expanded on that, but we are all pressed for time.. It's best to save these things, and work on them when you have time, rather than releasing whatever you have as soon as you think it's presentable. This was good, but for a flash to become a masterpiece, it needs to have more plot, and length.
Best of luck of future flashes,