I started stuttering in my native language

I started stuttering in my native language I'm Dutch, 16 years old, and I'd been going to a Dutch school for my entire life until this school year. I changed schools to an International School so now all my classes are in English. I'm fluent in English, native in Dutch, and I've never had a stutter before ever. Now I'm noticing that whenever I speak Dutch with my parents and old friends I stutter and find it difficult to form sentences. I don't stutter when speaking Dutch. What causes this?