The technical term that you are looking for is masturbation. And the thing to remember about masturbation is that we all do it. Yes, absolutely everyone. (Even me - especially me! lol). Quickest way to spot a liar is to catch someone denying that they have a go occasionally. Its the most normal and natural thing in the world. After all, who knows your own body better than you? It stands to reason you are going to bring yourself pleasure better than anyone else ever could. So trust me, there is nothing at all wrong, dirty or even naughty in what you are doing. We're all at it!
Nor is it all that surprising that you feel particularly in the mood after working magic. Magic can involve a lot of mental effort and any form of sex can be a release after prolongued mental work. It does not mean you are performing dark magic, it just means you are working your brain hard and desiring a more physical type of effort afterwards.
I can't tell from your question whether you worship deity or not but I'm going to gamble that you do as your concerns suggest you feel you are being judged by someone. I can't speak directly for the God and Goddess but what I can tell you from a purely common sense approach that I very much doubt they would mind you doing something that causes absolutely no harm at all and brings you pleasure. Why would they mind? Why would anyone mind? If we weren't designed to enjoy a bit of fun on our own we more than likely wouldn't have been designed to have such easy access to our fun bits.
Incidentally I wouldn't worry about "accidentally" performing black magic. Magic is a neutral will dependent force. You can't accidentally intend something contrary to your own will. You can of course deliberately choose to use your own will for what is popularly termed black magic, but you certainly can't do that by accident. So stop worrying, enjoy what you are doing and relax x
image http://www.sxc.hu/photo/540757 (a kartha)