Being 250 feet tall, Runa is affected by the square-cube law. Being a Jotunn means that she's magic and can resist most of them. (otherwise she'd be crushed under her own weight). Every movement Runa makes is proportionately much slower than those a human makes, with far more momentum. She's used to it, of course. But, when she shrinks down to human size, especially if she hasn't in a while, she's not at all used to how easy it is to move around, so she tends to overshoot everything. Give human-sized Runa a cup of something to drink shortly after she shifts and she may just well splash the cup's entire contents in her face!
But what she finds most disconcerting is her heart rate. At her normal size, her heart beats with tremendous force; slow heavy beats that make an elephant's heart rate of 30 BPM seem rapid in comparison. For her to feel her heart pounding through her chest even at 120 BPM always makes her feel a tad anxious, like she's having a heart-attack, and no matter how many centuries she lives, she's unsure she'll ever get used to it.
edit: that's all some really good stuff!!! Definitely the part about the poison when she's angry, and relaxing in the jungle, too! And yeah, she doesn't talk about her backstory, she's yet to bring it up with anyone else in the RP heh (besides brief mentions of her wanting to learn how to kill a god)
(as an aside tho, I didn't really mention this in her bio but she's got a gameplay mechanic where she has to maintain a balance between tranquility and fury; if her tranquility is high she's got a decent defense buff but next to no damage, and if her rage is high, she's got higher than normal attack power but becomes super easy to harm. so the "anger so extreme it helps her in fights" is simultaneously both canon and not canon...?)