Hi Jean,

This is an educated guess, but I'm pretty sure it 
is from the end of Episode #73.  Usagi is upset 
because she can't transform into Sailor Moon and 
save the other senshi from being pulled into 
Rubeus' UFO since Chibi Usa has stolen her 
transformation brooch.  She tries to slap Chibi 
Usa, but Tuxedo Mask stops her.

Hope this helps!!
~Luna-Art >^.^<