I’m running the plugin Content Views Pro, which has an option in the settings to prevent Bootstrap from loading (I presume the option is there for cases like this when other plugins or themes load Bootstrap). I simply checked the box and refreshed and everything worked fine.
Check the HTML of your site in the browser to see if anything is loading Bootstrap more than once. It took me a while to figure it out, but I was using Firefox’s Inspect Element tool to explore what was going on in the CSS. It was very helpful.