She's a shih-tzu and in the last few weeks has started being aggressive towards myself and my daughter. When either one of us sit on the sofa/ armchairs, not always, but at times, she will jump at us barking and trying to nip- she usually gets clothes. When she does that to me I just remove her to another room until she calms down. When she does that to my daughter I get physically between them, towering over her, and then do the same removal from the room. She wags her tail the whole time she does that, so part of me thinks she sees it as a game. Before jumping/ barking she will just be asleep or just simply walking around- basically she is calm. She is very calm the rest of the time, sleeps a lot. She's walked daily for up to an hour, plus several times for a wee (no direct garden access).
Is it hormones? Play? Advice would be greatly appreciated.