It seems the possibility you have been hacked is quite high. As the Twitter Help Center Page states, here are some common things that happen when you are hacked:
- Noticed unexpected Tweets by your account
- Seen unintended direct messages (DMs) sent from your account
- Observed other account behaviors you didn't make or approve (like following, unfollowing, or blocking)
- Received a notification from us stating that "You recently changed the email address associated with your Twitter account." (even though
you haven't changed your email address)
The Twitter Support page states that you should perform the following steps to remedy the issue:
Change your password
Please change your password immediately from the Password tab in
settings. Please select a strong password you haven't used before. If
you can't log in to your account, please see this troubleshooting
Revoke connections to third-party applications
While logged in, visit Apps in your settings. Revoke access for any
third-party application that you don't recognize.
Update your password in your trusted third-party applications
If a trusted external application uses your Twitter password, be sure
to update your password in that application. Otherwise, you may be
temporarily locked out of your account due to failed login attempts.
Your account should now be secure, and you shouldn't see the
unexpected account behaviors moving forward. If you're still
experiencing issues, please file a Support request for assistance.
Along with doing this I suggest scanning your computer for malware using a program such as Malwarebytes.