The most common cause is that you had a discount offer before, and it went away! These discounts only last about two days, so they have to be used within that time frame.
If that wasn't the cause, however, that means you just saw us raise our prices. From time to time, we reassess the price that is appropriate for Brilliant Premium. We raise prices in order to address changes in currency exchange rates and to reflect the additional value that we provide to our customers. Even at today's price, we think that Premium is a steal :)