Dissociating happens for me a lot in therapy... I can understand why you feel embarrassed, but sometimes it's like an emergency shut off valve when your system is too overwhelmed. My therapist usually takes it as a sign that things are getting too much for me and she tries to back thing off a bit, help me get grounded. I think it is concerning what happened with your T and you might want to ask what her intention was. I think it's important in therapy to know you have some control of when things are too much and when to back off. Or it can feel quite unsafe.