My cat had a miscarriage, what caused it?

So my cat had a miscarriage she's not a year yet until october and this happend on april, what could cause her to have a miscarraige, she aborted 4 kittens they were all formed but didnt have fur small as a mice it was very sad, she acted normal then days after she was tired all the time and ok nly wanted to sleep, thabk God she's ok now, but im wondering what caused her to have a miscarraige?