You might want to change your password, especially if you use that same password elsewhere.
Details of the recent security breach on Business Insider. Supposedly, Dropbox is emailing those whose accounts are known to have been compromised.
Here is a handy page that will let you see what devices have accessed your account. (That link will redirect to the home page if you're not already logged into Dropbox.)
(h/t: Zscaler, via email)