Dog "forgetting" commands

I have a 2 year old Labrador x Border Collie mix and she has known the command for "lie down" since she was a puppy. But lately, she seems to have "forgotten" how to do it. I haven't changed the command signal and she tries to do something (like sit or give me her paw, etc) so she's not just ignoring me, but it's like she's confused and doesn't remember what to do. Is this normal?