Potty problems

So about 3 weeks after giving birth to my second child, my first born who is 3 years, stopped using the “big boy” potty and started having accidents. He’s been potty trained for almost 1 year now and hasn’t had any accidents not even during the night for about 6 months. I don’t know if it’s a jealousy thing but I don’t know how to fix it. I’ve explained to him over and over that he’s a big boy and he doesn’t need to have diapers anymore, I asked him why he wasn’t telling me or daddy that he has to potty and he’s saying that he just has to go so bad he can’t hold it. When he said that I thought maybe he has a bladder infection or something so we took him to the doctor, nothing. The only time he won’t go in his pants is when we’re in public. I’ve put him in pull ups for now but I feel like that’s just telling him it’s okay.

Has anyone else had this problem???