I've seen that look on many post-menopausal women. Because their vaginas don't have the same elasticity or lubrication as before it hurts a lot to get fucked. They just don't want to disappoint their partners by saying it hurts. But that is the face and the sound they make when they really aren't enjoying it. They'll let you know by not fucking you again for several months. What they love best is lots of foreplay with no fucking afterward.