Bulk email

I have 2 Yahoo accounts and email addresses. They seemed to work the same way, but for no reason, one suddenly looks different and says ‘spam’ on the left side, instead of ‘bulk’ like it had done before. The font arrangement is a little different too. It is still the Classic Yahoo, and I have not changed any options on either of the addresses. Does anyone know why this would happen, and what is the difference between ‘spam’ and ‘bulk’?