I haven't played in about 3 months and now when I try to sign in it says you cannot use another persons sign in ID or email why is that?
Every PSN account is associated to a XMB account
Your PSN account is what grants you access to Sony's online services [e.g. PSN store, online game-play, etc]
Your XMB account is a local account in the PS3's system. [e.g. User1, User2, etc]
Only one PSN account can be associated to an XMB account.
The message "You cannot sign in using another user's sign in ID" displays when you are trying to sign into the PSN using a PSN account that is associated to another XMB account. For example, if you want to sign into the PSN to play as "XH4KK3RX" then make sure you are under the right XMB account, the one you originally used to create the XH4KK3RX PSN ID.