I'd hazard a guess that you may have acquired some malware or a badly-behaved app. In the app drawer, is there an one for Downloads (icon is usually a down-pointing arrow)? If there is, tap it and it'll take you to the download location and you may be able to get a clue to what they are. A lot of apps will load graphics, but they normally do this silently and the first you notice is some odd images in the Gallery (nothing to worry about).
The first thing would be to turn the phone off, give it a moment and then restart it. Does the behaviour persist? If so, have you added any apps recently? If you have, uninstall them in reverse order and see if that helps. Have you installed any apps other than from Google Play? Be very suspicious of those.
You could also try installing My Data Manager and use it to see which app/s are using data intensively - that might also provide a clue. In it, view Apps and choose to display the last hour's worth.
If all else fails, you may need to do a factory reset (back up your content first).
