Why can I speak freely alone but stutter with others and in my thoughts?

Why can I speak freely alone but stutter with others and in my thoughts? When I’m home alone, I walk around the living room, talking out loud to myself. I say stuff like what I’ve done, what I’m gonna do next, my plans and lots of nonsense. It feels super normal, no issues there. I talk just fine, like a regular person. But, when I need to talk to a doctor or nurse, I just can’t. I start struggling with sounds like ‘rrr’; I stutter. It’s the same with phone calls. And the funny part? I try practicing these conversations in my mind, like when I’m sitting on the bus. Guess what? I stutter in my thoughts too. It’s so weird, I can’t figure it out. Is this even stuttering?