It could be, but honestly stuttering is weird about when and what conditions it happens in. I don’t stutter as much when I’m whispering, for one thing, and I have no idea why, I just know it’s a thing about my stutter. So while it could be a social anxiety thing, it could also just be a quirk of your stutter. Personally I’d try testing it out. Maybe record yourself talking when alone. Do you stutter then? What about making a phone call by yourself? Etc.