Why do I scratch my boyfriend in my sleep?

My boyfriend and I are close sleepers, and I usually sleep with my head on my shoulder and my hand on his chest. However about 4–5x in the past six months he’s woken up to me digging my nails into his chest. Hard.

This is completely unintentional on my part since I’m deep asleep when it happens and unaware of what I’m doing. However, his yowling is usually enough to wake me and I immediately feel horrible. Also, I don’t think it’s related to anything I’m actually dreaming about as I can never remember dreaming when it happens.

I did some Googling and some people say that this is a symptom of stress or even PTSD, which I have been diagnosed with due to traumas from my past. However, I haven’t found any actually credible information on this and would like to get to the bottom of it.

For the time being, I’ve cut my nails short so I at least don’t draw blood. :/