I have a lovely 4 mth old male kitten, had him for over a week, all fine, then he poo on my bed??

My lovely kitten is so sweet, he has been using the litter tray really well, i clean it out after he goes every time! then after having him for a week, I changed my bed sheets and he done a big poo on my bed???? is this due to a different smell on my bed (washing powder)????? as i said he has been sooo good, he has lots of toys 2 scratching posts, Please help me to understand my kitty x