I caught my roommate masturbating...to me?

She's been my roommate for about a year and over the past year we've become close friends. It's just been the past two weeks were we haven't been getting on so well. She'll snap at me over the slightest thing, when we do say something to each other. I'm not going to lie I've been feeling really hurt by her changed attitude towards me. We use to talk about everything and do everything together- I don't think I've ever met someone I'm so completely compatible with and for the most part, I know it isn't one sided. The night before last she started complaining about a bowl I had just put in the sink. I told her to relax it's not a big deal. And she picked the bowl up and dropped it on the floor, it shattered everywhere. I was pissed, we started arguing and I yelled at her pretty badly. A few minutes later she said she was really sorry. Okay, I was confused when she apologized so easily. Then I asked her if she was pmsing or something and I was kind of only joking when I asked her "goddamn, when was the last time you got laid...are you sexual frustrated?" I laughed it off, but I could have sworn I saw her cheeks redden. I though nothing of it anyway she's always been uncomfortable when it comes to discussing sexual matters. I'm a lesbian and she is (I'm not sure) were both single. I'm 24 and she's 23. Anyway, we made up, but I was still a bit upset and hurt from her behavior when I came in late from work last night. I don't know if she lost track of time, but I usually get home from work around the same time most nights. I didn't see her anywhere in the house, so I went to check her room. The door was slightly ajar and the lights were off. I didn't knock before I opened the door and walked in- we never do. I heard my name like a breathy whisper being called twice. I answered thinking she was in bed and sleepy, but when I flicked the light on...she was in bed alright, but completely naked and from what it looked and sounded like on the very brink of an orgasm! She leaped out of bed and sort of stood there frozen and so was I. I was in sock, but then she started to cry! I mean really sobbing hard, her arms wrapped around herself trying to cover her nakedness. I couldn't stop staring and my mind was spinning. After what felt like a decade I saw her towel on the floor and picked it up wrapping it around her. She was trembling and couldn't stop crying. I told her if she wanted I hadn't seen or heard anything, but when I looked down on the bed there was a photo of me. She saw me looking at it and went from really flustered to pale. Yes, I was really shocked to find her masturbating to me, but I'm not going to lie the girl is gorgeous. I would have been only very flattered and of course extremely turned on, bu her reaction to me finding her really worried me. She really looked mortified and terrified, then she started to apologize! I sat her on the bed, hugged her, held her and told her everything was fine. It was a long while before she calmed down. She had her face buried in the arch of my neck- she just couldn't look at me. I helped her get ready for bed and she let me stay with her last night. The thing is now today I'm not sure what I should say to her...I'm pretty sure if I don't bring it up she wont and it will only eat away at her and drive her nuts. And I'm sure when I do bring it up it's only going to cause new tears of fear and embarrassment. I don't even really know what I should say, how long should I wait to bring it up?! I do have feelings for her...a lot. I want to tell her, but I am still a kind of shocked- not the whole masturbating to me thing, but her reaction afterwords. Should I wait awhile before I bring it up...what should I do please?!