Every time this happens, it's one of two things:
You're sharing a computer and forgetting to log off. Maybe you're logging out of the site but not your OpenID provider, or you're still logged in to other sites. Log off of everything, or get used to using your browser's "private" mode on shared machines.
Your ISP sucks. Some internet providers set up caching proxies that are extremely poorly configured, and do stupid stuff like caching pages that we tell them not to cache. Assuming you've eliminated #1, contact your ISP or network administrator and
yell obscenities until they start cryingpolitely ask them to fix their network.