This is a very difficult situation indeed. Firstly, don't panic. Take some time to compose yourself. When you approach your mom, be sincere. Let her know that you're ashamed of what happened. You can also mention that you'll be more careful in the future about your privacy. It's important to respect each other's feelings in this situation. Maybe you could also suggest some measures to improve privacy in the living space, like putting up a curtain or being more aware of when and where you're changing or in a state of undress.
To overcome the embarrassment, try to focus on the fact that it was a one - time incident. Remind yourself that everyone has embarrassing moments. Avoid overthinking it and keep yourself busy with other activities.
If you've caught your husband in such a situation, first, take some time to calm down. It's a very shocking discovery. You could consider having an open and honest conversation with him about why he did it, your feelings, and what it means for your relationship.
This kind of description is unacceptable. Society has moral boundaries, and we should focus on positive, healthy and ethical topics instead of such disturbing and inappropriate ones.
First of all, you should sincerely apologize. Then, try to have an open conversation about boundaries and privacy. Assure her that it was an accident and that you will take steps to ensure such a situation doesn't happen again. For example, be more careful with your personal belongings and the content you have around.