What Could Be Going On With My Son

My son is 15 almost 16 and has been acting very weird since he started going to his dad's house. His dad has a new girlfriend who has a 23 year old son who lives with her and my ex. He's super lazy and won't get a job. My son has been going over there for a few months now and he's started acting weird. He started randomly wetting the bed. I asked what was going in and he just gets super stressed out and tried to change the subject. Also Everytime my ex's girlfriends son is around my son gets super nervous and looks like he's having a panic attack. My ex's girlfriend called me saying her son wanted to hang out with my son. When I asked if he wanted to my son starts crying begging me not to make him go. My son almost NEVER cries. I was shocked. I asked again what was going on and he said he was sorry and it was nothing. I'm so confused. I asked my ex if his gf son could be doing something to upset our son and he called me a jealous bitch trying to pin my son's emotions on his future wife's kid. He's making me feel like I'm crazy. My son randomly starts peeing the bed, acting super strange, and has a panic attack at the thought of being around her son... Am I just being stupid?