I have to tell you OP that it sounds like she is into long edging and doesn't enjoy the sex with you as much as the edging she can perform on herself.

I would put it to her this way "I could care less whether you masturbate or not, however when I catch you at it and you lie to me I can't accept that. If you lie to me about it once more I hope the lie is worth the potential future of our relationship because we will not have one. Do you admit that you masturbated in our bed before I asked about how the orgasm was?" and stick to the threat.