Would you please tell me what the difference is between these two sentences, and if they can be used interchangeably?
You have become weird.
This means that the person used not be weird, but these days he/she is weird. It suggests that this is now a regular personality trait of this person. They are now weird all/most of the time.
You are being weird.
The person is behaving weirdly right now. Perhaps they weren't doing so five minutes ago, and they might not be doing so in five minutes time, but right now, at the time of the statement, that is how they are behaving.