Issues after logging in

I was in the back-end of the site making updates. Everything was fine, until I clicked “update” on the page I was working on and then the entire back-end interface turned into a text-only version of the front-end of our website. But the URL in the URL bar is still the wp-admin login URL.

I cleared my cache and tried again, which worked, but half an hour later I had the same issue. The issue happens in firefox, Chrome, and IE, and restarting my computer didn’t do anything. Any ideas?