Shock Value
The animation was jerky for most of the movie, unfortunately. While I'm not an animator, and hence the product of your work is infinitely better than anything I can(not) make, it doesn't take an artist to see this isn't the highest quality animation. Definitely not bad, when compared to a lot of junk I see, but also not very good.
What really makes me score you lower is that there is absolutely no story here-- it is shock value for shock value's sake! Gruesome and disturbing images does not a story make. I mean, I would expect that kind of story from a 3rd grader at a campfire, "A bug went into a basement at the circus, and there was a mangled clown shackled to the wall, and uh, the bug climbed in a whole in the guys leg and wound up in the guys neck and then, umm, the bug crawled in the clowns mouth, and guess what! the clown ATE THE BUG!... oh yeah, then the bug's partially masticated body was spewed out of the clowns chest cavity onto the floor... and uh, another bug ate IT!" Seriously?
The music fit the goal of the animation perfectly-- I felt a chill shoot up my spine in the first few seconds of the animation.
I think your animation and sense of horror are on their way, especially since this is your first Flash animation, but you need to work on plot for your next piece of work (which I hope you will make-- you definitely have potential).