You are probably looking at the Universal Inbox.
Tap the Mailboxes button at the top of the email list. You will see two lists. One for just inboxes and another for accounts. At the very top of the Inbox list you'll see the All Inboxes choice. All new emails from all accounts will show up here.
To see just your emails choose the inbox for your account, or choose your account from below. The first choice just shows you new emails. The second will take you to a list of all the folders you sync with that account, and let you choose which to view.
When creating a new email the address associated with the account you are viewing will be used. You can change this by taping on the email in the from field and choosing another account's email address.
When you are in the Universal Inbox it uses the email address set up as default under Settings >> Mail, Contacts, Calendars.
The Universal inbox was added with the last update.
