What Causes The Child To Be Defiant And Short Tempered?

i am a grandmother of a 5 year old boy.. his is also in my classroom (pk4) He has always been impulsive, active (most likely adhd) but in the last three days i have seen a huge change in his behavior... defiant, hitting out at other children, angry... can a child jump from a toot to that angry (even if he might have Occupational Definace Disorder not diagnosed just a thought) . but does it normally go that quickly?