As of today, my account on Pixiv just got suspended. They claim that I, "Used existing images without permission from the creator and/or subject (model)", which I didn't. They don't link the exact work of mine that was breaking this rule, and now I can't upload new art or edit it, and can't message people. I don't know when it will be unsuspended, but, as of right now, it's horse shit.
Yatsufusa
It's possible that somebody uploaded your work to somewhere else and claimed it as their own. Deviantart.com is famous for not giving a sh*t about ownership and is easily swayed by sock-puppet accounts and such. Fun times...
Follow Pixiv's procedure to get recognized as the rightful owner/original artist. I'm sure they must have an extensive wall of text about how to do it 'somewhere'. In doubt, creating a new, original piece and showing it to 'them first' before publishing it will probably help.