UNSAFE lock??

'Last night'(the correct statement is THIS MIDNIGHT),around 3am,I only took my bath.(I know it quite late).When I finished my bath,and wash my clothes.Suddenly got someone need to use bathroom.She strongly open the door and she really can open it.It scared me.I already locked the door before I taking my bath.Why she still can open the door???Luckily I finish bathing and wore my clothes,if not....