At first you were a kid, but when you turned 11/12 you started puberty. Maybe you haven't noticed that much yet. Others can actually suffer a lot from it. Anyway, every person goes through this phase. Your puberty lasts on average from the age of 12 to 18, but this varies per person.
Hormones are a major reason for all the changes in puberty. Hormones are substances in your body that act as regulators. Is something dangerous happening? Then your hormones help to respond to that. Have you not eaten in a while? Then hormones make you hungry. Your hormones are made in all different places, in endocrine glands. You can see that in the picture below. During your puberty, a lot of hormones are released in your body. That's why you react so strongly to things (even when you actually know it's not that bad).
So a lot happens to your body during puberty. One of those things may already be in play:
The growth spurt
A growth spurt is a period in which the body suddenly grows very quickly. Such a growth spurt takes place in puberty! That's why some boys in 1st grade were shorter than the rest, but those same boys in 3rd grade are suddenly the tallest in the class!
After the growth spurt you often move a bit sloppily. That's because your muscles grow later than your bones. Immediately after the growth spurt, your muscles are not yet strong enough for your longer arms and legs.
Until puberty, boys and girls grow at the same rate. During puberty, girls start their growth spurt earlier than boys. The growth spurt in boys is stronger than in girls and often lasts a little longer. Boys are on average taller than girls.