You haven't been yourself lately. - This is a rather common expression. You're not being yourself lately. - I've not ever heard this expression. I would not use it.
Time expressions must be used correctly according to the tense. Lately, recently, often, usually, etc are typically a sign of the present perfect contexts. So you haven't been yourself lately is perfect English. However, same sentence but with "today" after yourself, is the substandard variety. So, you may say " you re not being yourself", or "you are not yourself today". Both are grammati