twitter won't let me change my username?

[Maiya]

So I have been wanting to change my twitter username (@___) for a while, but I haven't been able to because it keeps saying "Your username can only contain letters, numbers and '_'", and it has been saying that since I started to change my username about a month ago. I started this account around September 2012 and have never changed the username before, so I wouldn't think it was because I have a username-changing limit or something. I have tried countless words and phrases with underscores, numbers, and capitals and lowercases, but that message keeps coming up and redirecting me back to the same page. I have also tried making accounts out of a lot of the usernames that wouldn't work, which worked fine.
I know that you have to go into your settings, click the "Account" tab, and can change it right there where it says "Username", and I know that all of the usernames I tried weren't already taken.
Any ideas? Thanks )))