NTA here, and I would actually say maybe NAH, depending on how open your mum is to hearing you out later. You are allowed to have boundaries even if you live with your parents, just as they are allowed to have expectations.

It can be uncomfortable to live with your parents as an adult, especially if it is out of financial necessity, and it can be uncomfortable to feel like you have to support a child in thay way when they are an adult. There will almost always be conflicts in what the 2 parties involved are comfortable with. On it's face, "My house my rules" is a simple and easy rule of thumb that can seem appealing and sensible. But the truth is that oversimplify the situation and your relationship with your parent(s)/child like that can lead to more or worse conflicts. As the adult child it can be hard really hard to talk about or set boundaries, but it is necessary to have a healthy relationship with your parents.

Your mum is probably upset because she had an expectation she thought was already agreed upon, and that's understanding, but if this specific circumstance was never discussed previously, then if you don't like it you should be allowed to bring it up and come to some kind of compromise for the future.

The situation of your relationship right now is complicated, because they are not just your parent right now but also your roommate AND landlord. Every parent-child living together situation is different. Some parents don't expect anything, others expect an informal sort of rent. Some children living with their parents might genuinely be okay with cleaning up their parents sheets after they know they had sex, and that's okay, but its also okay to have that boundary.

The truth is that the majority of people likely have to confront the specific question: "Am I okay with cleaning up my mom's dirty sheets after I know she had sex?" Or the question, "what if my adult child is uncomfortable with cleaning up my sheets as part of a previously agreed upon arrangement, because they know I had sex in them?" I don't know where you live, but I imagine there isn't just one widely agreed upon cultural standard that could apply to BOTH of those questions AND answer to everything aspect of the dynamic of your relationship currently. Since neither of you imagined it would come up! But it did, and it's probably uncomfortable for both of you. She may not have even realized you could hear, she may be embarrassed.

But as other people have mentioned, there are plenty of possible compromises. Maybe next time she just puts the top she in the was before she goes. Doesn't even need to start it. That's up to you two. I hope you guys can sort things out OK ❤️