why does tumblr ask me to sign into my account from my blog?

[NKluv]

it's been happening since 3 weeks ago. i thought it was just a glitch, but it won't go away. my theme has changed, too. when i went to sign in from my blog, a little warning popped up & said that if it's not from the tumblr url i can get hacked. what should i do?

[Jessie]

You have probably been hacked, change your password then do the following:
Type in http://www.tumblr.com/themes/recover
Choose your blog.
Click view Backups.
Revert the most recent blog change.
then you’re all good to go