Help, why is this happening??

So my 3 year old son (almost 4 in February) has been fully potty trained since he was 19 months. Well a few weeks ago he started to jump when he has to pee and he won't go. He will just pee himself and admit that he did and just keep changing clothes all day long. He will be right next to the toilet in the bathroom just jumping because he has to pee but he waits to long. I'm curious on why he is doing this. Any advice or same scenarios ????