ExSb, it could also be your ISP expiring the DHCP lease either manually or setting an extremely short validity duration. They sometimes do that when they are trying to squeeze more customers into their finite IP address allocations.
I know that when my internet connection drops when logged into Pprune I always get logged out after I re-establish the link. This is because the reconnect gives me a new IP address and Pprune rightfully thinks I'm on a different computer (actually your old pprune session gets orphaned but eventually expires from no activity).