Ok. First issue: you appear to have done all your custom styles on Twenty Fourteen, and not a child theme. This means that the next time you update TwentyFourteen, you will lose all your custom css.
Here are instructions about how to create a child theme: https://codex.www.ads-software.com/Child_Themes
I’ve never seen this issue before, and I’m not seeing anything obvious to cause it — but I’m not a real CSS ninja. Sorry.
After you create your child theme, reinstall a fresh version of TwentyFourteen and check for this issue. If it disappears with a fresh version of TwentyFourteen, then you know it is something in your custom styles that is the conflict. If it still appears, then you might have a conflict with a plugin…
Of course, you could just start with testing for a plugin conflict:
Try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.