Who owns the TV?
What rules, if any, were stipulated?
So if you own the TV, then you could say "I am watching my TV. You have one in your room."
If it is hers, then say "Daughter, as soon as I am done watching this movie, you can watch it. It should only be 30 minutes more."
You could go now to her and say "Roomie, what was up with trying to guilt me off the Tv the other day? I told you I had 30 min more. Yes I was writing a quick email at the same time. The TV is in the LR and no one person has control over it. First come first serve we agreed. Please don't put your daugther in the middle. She could have watched cartoons in your room if necessary. If there is some other issue let me know.