I have been seeing my psychologist for 3 years and we have a good relationship. I have found that when I notice mine smiling or grinning, I simply ask her what she is smiling about and she is able to tell me why she is doing it.
Mine have been for reasons like she finds some of the things I say amusing, or she has to smile at the way in which I sometimes verbalise things that irritate me etc.
Asking your psychologist is the only way you will know what she is thinking at the time, so just ask her when it is happening and see what her response is.
Des