I admire your honesty in posting this. Masturbation can be healthy when it doesn't include pornography or fantasy, and you are the only one who can decide when it's healthy or not. It might be helpful at this point to clearly define your sobriety using the Three Circles from SAA.
Sex addiction is different than most addictions in that sobriety is not as black and white. Having this in writing can help remove some of the anxiety about understanding what is and isn't healthy, and can even give you ideas about healthy "outer circle" activities to do when you are struggling. I would suggest writing your circles with the help of another addict or someone who knows about your addiction.