my 3 year old who has been Potty Trained for 4months keeps having accidents !

So my son just turned 3 last Friday and for the most part his speech is really well. He knows how to saw I got to go pee or poop. But there are also days where he won't say a thing and he pee's on himself. And when I ask him why he didn't say he acts nonchalant. I am starting to worry because he will be starting preschool soon and I can't understand why he keeps peeing on himself. Anybody else having or had this problem and with a toddler?