If you had a Level-Down, it means that the sum amount of Artist XP you gathered over the past 10 weeks, after applying the fading system, was not enough for you to maintain your current Level, dropping you into a lower tier. Note that the calculation of your Artist Level happens weekly - every new week, you might fluctuate between Artist Levels relative to the amount of XP you currently have. This is natural and is the intended system.
We apologize if you had a Level-Down, but as long as you continue streaming and doing your best, I'm sure your Fans will grab you enough Artist XP for you to climb back up!