How do I talk to her about this?

I have my own apartment which

is small and I have a roommate who I’m really close to. we each have our own rooms with one common area. She has her boyfriend over a lot which is fine as long as he doesn't overstay or anything because our apartment is small and her hometown is over an hour away from our college.

he came over and they cooked dinner and made a pie. My roommate called me from my room which is off the side of the kitchen and living room and asked if I wanted any and I told her I would come get some later. About an hour later after I took a shower, I came out my room to get some food and was wearing spandex cheer shorts and a graphic tee and a headband. My makeup wasn’t on and I didn’t have a bra on or anything. Her boyfriend walks out of her room at the exact same time and It was really awkward and I was like “I’m sorry I’m not dressed” and kind of shielded myself. He stared for like a second and then shut the door without saying anything. I thought that was kind of rude and I haven’t seen him in weeks and of course that’s when I saw him again. I also kind of got mad at my roommate for letting him walk in our apartment like that by himself. I went back in my room afterwards and he came out like a few minutes after but I think my roommate was with him. I heard him yell something which sounded like my name but I’m not sure. I’m probably making it out to be more than it is, But it was so awkward. I like to look my best in front of everyone. I told my parents on the phone and they said I should have a talk with her about it and boundaries and stuff.
I think I should too but I’m such a non confrontational person and I would feel like a pushover if I didn’t say anything and I need to start standing up for myself. She asked me to help decorate our Christmas tree today and is acting like nothing even happened but it’s kind of bothering me. What do I even say about it?
How do I talk to her about this?
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