Why does my iMessage say I'm sending from my email instead of my phone number?
Assuming you did not change any other settings after the upgrade (like Restrictions), your iMessage may need to be reactivated. When you look at Settings > Messages, you would also see that it displays the message "Waiting for activation...".

There are several ways to get it activated, and sometimes it may take a few tries since Apple's servers may not respond in a timely fashion.


Turn off iMessage on Settings > Messages.
Turn off your iPhone (hold the power button until the slide to power off prompt appears and slide to power it off).
Turn on your iPhone after a minute or two.
Make sure you're connected to WiFi (usually more reliable than Cellular).
Go to Settings > Messages and turn it on. Accept the prompt for the warning that carrier fees may apply for the activation SMS.




If the above does not work, you can also follow the steps from iOS: Troubleshooting FaceTime and iMessage activation (edited steps below):


  If you see "Waiting for Activation" when trying to activate your phone number with iMessage or FaceTime on iPhone
  
  To resolve this issue:
  
  
  Ensure that you have an active Internet connection. You can complete activation using Wi-Fi or a cellular data connection.
  Ensure that your iPhone is set to the correct time zone: tap Settings > General > Date &amp; Time.
  Note: If Set Automatically is on but the incorrect time zone appears, turn Set Automatically off and choose the correct time zone, date, and time.
  Verify that you can send SMS messages. You need a valid SMS messaging plan to activate FaceTime and iMessage.
  
  
  If "Waiting for Activation" still appears after you perform these steps, turn FaceTime and iMessage off and on in Settings > Messages and Settings > FaceTime. You may need to wait up to 24 hours to complete activation. Most attempts will take less than an hour, but some attempts may take up to 24 hours. If your issue isn't resolved after 24 hours, please contact Apple for assistance. If you've recently performed a remote wipe on your iPhone, wait 24 hours from the time you performed the remote wipe, and then turn iMessage off then back on.