Inspired by the “Discordant Harmony” episode, where Discord almost disappears because of lack of using chaos magic, I thought it may as well have a similar effect on Freya. Although she doesn’t suffer from the threat of essentially dying, the chaos magic does build up inside her and when it becomes too much for her to handle, she can become quite aggressive and feral, with bouts of chaos energy forming around her (like electricity) and turning any object within close range into anything that resembles something chaotic.  
She’s usually careful and aware to create a few chaos spells here and there, mostly in her gardens, creating wicked and enchanted flowers, but she can sometimes forget to do it enough and doesn’t realise it until someone tells her, how her behaviour has changed (angry outbursts, shouting, snarling etc.)  
The biggest giveaways, besides the behaviour, are the extra smaller horns, slightly flowing mane and puffed up fur
 
Bonus:
 
-She needs to balance out her magic usage, so that at least 50% is chaos magic  
-When in this state, she speaks less and uses animal sounds, mostly predators  
-Fluttershy and Discord decided to call this state “overflow”  
-She used to have more overflows as a child, since she couldn’t control her magic well enough and refused to use either normal or chaos magic (luckily, aunt Twilight is here to help)