Husband snap chatting baby mother !

Ok my husband been borrowing my phone cause he lost his. He came home from work and o ask to use my phone, now I'm doing my normal surfing when I clicked on my snapchat app. His account popped up and there was a snap so I clicked on it and their was my husband baby mother sending him a snap like she was posing. So I confront him about it and he tells me that it's none of my business and that he is grown and he doesn't have to explain anything to me. Should I be suspicious? I mean what do they have to snapchat for? The baby is always at the grandma house? I'm just pissed off and he act like it was normal to do.