Hi T.,
I am not an expert on the common underlying emotional or psychological issues that might cause a child of your daughter's age to use this behavior to express anger. But I been through some similar issues with my son's behavior in the past. I do believe that we need to teach our children how to deal with their anger and how to express their feelings in ways that are not destructive or inappropriate, just as importantly as we teach them manners and respect.
I tend to lean more towards the idea that your little girl simply thought at the moment that peeing on the floor of her room would make you unhappy and she was unhappy so she did it. Most 7 year olds don't think too deeply or long about their actions yet, they mostly just do what occurs to them at the moment.
I think it's great that you talked to her about it and she told you why she did it. When a child tells you they were angry, that's the perfect opportunity to talk about anger and discuss some constructive ways to handle that emotion. If more parents would acknowledge anger as a natural reaction and give their children positive ideas for how to manage their "bad" or hard feelings I think we'd have a lot less violence and self-esteem issues in this world.
If they never learn how to deal properly with anger they will continue to act out when they are mad. Heck, most adults these days do not handle their anger appropriately. If we yell and lose our temper when we are mad, our kids think that is how you are supposed to act. Or they pick some other action that occurs to them if they don't know what to do.
Talking about anger is a good thing. Ask your daughter what she thinks would have been a better way to let you know she was angry, so you could help her not be. Once you've both come up with a few better ideas, then you can also talk about how unsanitary peeing in the floor is and how you know she is a big girl now and that's not a choice a big girl should be making. When you let her know that you believe in her ability to make good choices she will be proud to rise to your expectations. Be sure to praise her and look for times when she does make good choices. Best of luck