Twitter is updating the @Replies (@your_username in the right side menu) which now becomes Mentions. That means that from now on, under the @Your_username tab you will see all the tweets that include your @username opossite to before, when it used to show only @replies that started with your username.
Before, under @replies:
@username <text>
Now, under Mentions:
<text> @username <text>
So now, no matter where your username is mentioned in a tweet, it will show up under "Mentions".
So now, no matter where your username is mentioned in a tweet, it will show up under "Mentions".
