If you haven't done this, you can add/edit notifications in Calendar settings. The default is a popup notification 10 min. before the event so I suspect you haven't edited this.
- Go to Google Calendar.
- Click the Settings gear at top right.
- Choose "Settings" in the dropdown.
- Click the "Calendars" tab (top left).
- Look at your list of Calendars. On the right you can "edit notifications" for each calendar.
In here you can Add/edit a notification, choose to get an email or sms when someone invites you to an event, receive a daily agenda etc.