You don't say if your calendar app is an app or a widget that works off Gmail's calendar. If it's a widget, you might consider uninstalling the widget and then re-installing it.
Have you tried a battery pull? Take battery out, wait at least 60 seconds, put battery back and turn on. This will take care of all kinds of glitches (the X will self-diagnose). It isn't something you need to do on a routine basis, but whenever things start getting weird, it's a first option.
Also, what new apps have you installed lately? It's not unusual for apps to conflict with each other or with the operating system - or just be a bad app. If the battery pull doesn't work, try uninstalling the most recent apps one at a time and see when/if the problem goes away. Then re-install one at a time. If you identify one that is the cause, it should become permanent history at that point.