This also sounds strange to me, I would definately ask your T about this...or see if it happens in the next session at all and if it does then point it out as it happens and address it then.

I know Protocol brought up mirroring which does happen but mirroring and mimicking are different. Mainly mirroring is about reflecting the same posture or position...for example if a client sits with their legs crossed or facing a certain way the T mirrors this to try and make the client feel more at ease with them, it doesn't usually refer to copying a clients facial expression though, if a client is upset the T shouldnt look upset too - that would perhaps make a client worried the T cannot cope with the situation.

I think asking your T is the best idea