Try looking in the iPad "Settings" app that came pre-installed on your iPad. In there, go to the "Mail, Contacts, Calendars" tab. In the Calendars section at the bottom of that, check which setting you have for "Sync". It may be set at "Events 2 Weeks Back". Change this to "All Events", and let us know if that has made any difference.
If that doesn't work, it is likely to be a setting you have on your PC, such as what type of email account you have.