Investor sentiment turns sour after a bad month for stocks - here's why that's a bullish sign.

• The August sell-off in the stock market has flipped investors from bullish to bearish.
• Various sentiment indicators have flipped negative, signaling that investors are still worried about a broader stock market decline.
• But the rise in bearish investors is actually a contrarian signal that suggests the bull market has room to run.

There's been a lot of whiplash in the stock market over the past month, with the Dow Jones Industrial Average going from its longest winning streak since 1987 in mid-July to a near 5% sell-off in the month of August.