Why do I stutter in my native language, but not in English?

Why do I stutter in my native language, but not in English? I know it's not a speech impediment because I didn't use to stutter as a kid when I spoke Spanish. From high school to early college is when I would start stuttering here and there. It's not a severe stutter, but I do mince my words enough that my friends egg me for it. However, in English I never stutter unless I can't remember how to pronounce something. I've been speaking English since I was 6.