Yes it's my understanding that disassociation is very common with ptsd. It's a coping mechanism when fear/memories/feelings become completely overwhelming. It's also kind of the reasons that treating ptsd is so hard--it's hard to deal with and process stuff you push away so thoroughly.
Sorry for the long answer, but yes, it is common in ptsd. If you're seeing a therapist please do mention it to them--there are some ways to help with this response and still keep from being overwhelmed.
Virtual hugs coming your way