ATTENTION FOLLOWERS!!! Do NOT click the RESET PASSWORD button if a security warning claiming you’ve been posting suspicious activity pops up. IT IS A HACK/bug. Changing your password will not stop the message from popping up.
It looks like this:

IF you do happen to click it, ignore the emails to reset it that the thing sends you and do it all manually. Google “tumblr sign out”, then click & sign out that way. Then go to https://tumblr.com/forgot_password and reenter your old password. (Make sure the S is at the end of http. Important.) Then log back in and it should work. NOTE: I also did this on a different browser. I was using Firefox when the page popped up and it wouldn’t work when I reset my password — but when I did it all manually in Chrome, it worked. Sorry guys. Pain in the ass, I know.