I space out a LOT in therapy. I think I have spent at least half my sessions talking about how embarrassing it is for me! But I have learned a lot through those incidents--getting scared, checking out, and then learning to talk about it with my counselor. Now, I can tell him I'm dissociating when it happens, and afterward I can talk about why it happened. It is much less uncomfortable for me and the shame is decreasing.
I think it's pretty typical to be ashamed of this, but there's nothing shameful about it. Dissociation is just one of the post-traumatic responses. It's like a little internal alarm system that something is getting too close to trauma thoughts or feelings. Working through it in therapy will help you understand a lot more about what happened and how you have developed ways to cope with it.