history: I knocked on a closed bathroom door and no one responded. But, when I opened the door, someone was using the bathroom.  I can't believe I walked in on someone using the restroom.  Oh my_comma_ that must've been an embarrassing situation!
current: It was! I knocked first and no one answered, so I thought it was okay to go in. 
next: That's ok, you did the right thing. It probably happens to lots of people.