Need Help! Lose about 10% Mastery when entering combat

[Elliemintal-stormrage]

I noticed today when I was playing my elemental shaman alt that she loses almost 10% mastery when I go into combat.
I can not for the life of me figure out what is going on.

[Gungarg-uldum]

so is this the character that you are talking about or is it a different one.

[Nootzio-haomarush]

If you use an alchemical chaos flask it will randomly select a new secondary periodically altering your secondary stats. This the only thing I could think of as a possibility

[Corpsegrindr-nerzhul]

Take your flask off and try.

[Lyrasaya-wyrmrest-accord]

Check your trinkets?