Dental assistant tries to use the smallest tray that will do the job because an over large tray will extend too far back and make you wretch or gag.
Often the contour of the bone under the gums is such that a very slightly too small tray will dig into the gum. (S)he was trying to make you more comfortable, not hurt you on purpose.
At the most, your gum was scratched. It will heal quickly.