basically, a feeling of heavy sleepiness is a phenomenon that psychotherapy patients can experience when confronted by material that they are very disturbed and anxious about....falling asleep is a way of shutting down...and as such is a good defence against feelings of terror or high anxiety.
when a counsellor experiences that, it can be as a response to some sense in which what is being talked about is 'dangerous' to the client....it is a difficult thing to explain, but it is as if the counsellor is having the reaction 'for' the client...and it gives the counsellor a big clue that whatever is being talked about, however innocuous it may or may not sound...is a big area of anxiety.
Deep trauma is what i'm thinking of, that hasn't been allowed to come to the surface....
I'm not at all saying that is what happened with you, but I'm explaining that it can and does happen. I've experienced it as a client (a huge and heavy feeling of wanting to fall asleep when I knew that i wasn't tired before the session) and as a therapist (I have had a couple of clients that I realy really struggled to stay awake with at certain points in the session...and it was absolutely not because they were boring me...both had been seriously neglected/abused in very early childhood).
I think even if this woman was being just a common or garden git that shouldn't be in her job....taking it to her as a first step is the right one. But i totally understand if you don't feel comfortable with that.