Bathroom troubles

Okay so the only way to get to the bathroom is through my or my moms room. So my grandma goes through my room to use the restroom. Which she had just done less than an hour ago. So I figured I was safe to fool around a bit since she had just gone less than an hour ago. Well she knocked and I yelled WAIT! Not "dont come in" just WAIT but she didnt and walked in on me! So whos in the wrong here?! Because everyone in the house is mad at me for asking her to wait for less than a minute just so I could stop what I was doing! I feel like I shouldnt have everyone upset at me for just asking for her to wait! Like stop doing the damn *knock knock" come right in and actually wait when someone asks you to! This rarely happens because I normally can time it based on when she went last. Idk im pretty irked right now.