Maybe you have a psychogenic stutter, though from what I understand those are kind of rare. I'm not an expert, but I think it's pretty normal for one's stutter to vary depending on circumstances. If you have time, here's a really interesting podcast with Dr Soo-Eun Chang, one of the foremost researchers into what causes stuttering: https://stuttertalk.com/what-causes-stuttering-with-dr-soo-eun-chang-568/