Yes, it's a stim. Although it's not one of mine, I do understand, because I have several others that I do. And yes, I do often go deep into my own thoughts. Sometimes I am in one of my daydream story universes, which I have done all my life. Other times I am just zoning out because I am tired. Then there are other times when I am just mentally wandering around inside my own brain, as I think about stuff. Just like having a real, 3 dimensional living room, I also have a mental, no dimensional "brain" room.
I have sometimes received comments about my stims, but hey, it doesn't bother me, and it's not hurting anybody, so I will keep stimming, as it seems to provide some sort of needed outlet. Besides, every move you make burns calories, and I need all the help in that department that I can get. 
_________________
If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau