So I've had my beardie, Mohinder, for a little under a month now and he's had a perfect temper. He didn't have any major relocation stress, lets me handle him, doesn't hiss or bite, and his beard and belly are normally non black/stripey. However, today he got angry and puffed out his beard and made it black and made his belly really flat and wide and started hissing at, and even bit, my 4 year old sister. The first time, it was the morning and my sister was looking in his viv and talking to him, when he saw her and reacted that way. Then later, my friend and my sister were holding him while I scrubbed down his viv and rock and stuff, and he got like that again, and that's when he apparently bit her. I wasn't there for when it began because I was cleaning his tank, but it took him a few minutes and my sister leaving in order for him to calm down, but then he was fine around my sister for the rest of the day and is sitting on my chest, all normal and chill, as I type this. He's been around her pretty much daily since I got him and hasn't shown any signs of aggression towards her until today, and when she was holding him last night she wasn't unusually rough with him or anything. Any idea what it might be?