There are a few problems that happen with the Minecraft servers from time to time. To see if what you're experiencing is a problem with you are the Minecraft servers you can use this site. http://xpaw.ru/mcstatus/ I'm not sure if the site is allowed though, because it's an external link. The site will tell you what servers are down and what errors they may be giving. Another problem that you may have with using a shared account is "Bad login." Bad login can also be a login server problem, but if you do get it, just try signing out then back in. If someone logs into the account after you then you will get a bad login and have to log back in.