Pulling together a few different thoughts in some of the replies here, but either one of two things is going on. Either she feels some shame about telling you about her masturbation habits, or it's happening in her sleep and she doesn't know.

Years ago, I dated someone that denied masterbating when we were early in the relationship. It took almost a year before she opened up about it, as well as some of her other sexual desires. She wasn't comfortable talking about it, and I learned later, the more I pushed her on it, the less likely she was to talk about it. Your GF may not be ready to share that part of herself with you yet (even if it does turn you on) and that is totally ok and within her rights. Yes, it then gets weird that she's doing it in bed next to you, as it's also fair to know if someone is performing a sexual act on themselves in the bed next to you.

As others noted here, she may have some sort of sexsomnia. If she hasn't had a long term partner that she sleeps next to before, this may not have come up before. This also may have just started happening more recently. Also, she may even have some suspicions around this, but has a lot of shame coming to terms with it and/or admitting it.

The issue is now on the table, so you need to address it, but very delicately and respectfully.

"Hey, GF. I want to talk about something a bit awkward. I know we've talked about the masterbation thing before, and I don't want to rehash the same thing over and over. That said, I feel like we need some clarity about this before we go forward, as it's become a weird thing hanging over our relationship. I want you to know I'm not judging you for anything, and nor do I want to put you in a position that is uncomfortable. That night, while we were in bed, you were masterbating. It's happened before as well. Now, it is totally possibly that you were doing this in your sleep and didn't not realize you were. This is a thing that some people do, and from what I've been reading up about it, they can't control it and often have no idea they are doing it. If that's the answer here, again, I'm not judging you for it nor should you be ashamed about it. You may want to talk with your doctor about it, as sexsomnia is a type of sleep disorder, and it would be good to not only get an answer as to what is going on here, but also to make sure it's not a sign of some larger sleep or health issue.

That said, if what's really happening is you are getting yourself off and just don't want to tell me about it, that's ok. You have your right to your privacy, and I completely respect that. However, I also have the right to not have someone perform a sexual act while sleeping next to me. I'm not going to ask you any more about it, but I must ask and insist that if you are going to, please don't do it while I'm sleeping or trying to sleep next to you. This whole thing has left me feeling very awkward, as I'm sure it has you, and I think the best thing would be to keep that solo time 100% solo. Again, I love you and I care about you, I respect you privacy, and most of all, I'm concerned for your health and our health together."