How bad would one bad test score look for all scores schools?

I plan to apply to Carnegie Mellon, which is an all-score school. I did pretty badly earlier today, and I'm expecting something like a 31 - 32 in math, 34 composite. Considering that the average math score for CMU is 35, how badly would this look on me? Would it have a large effect even if I take the test again and get a much better score?