Your testosterone levels increase while you are on steroids as well as your estrogen. These are the sex hormones. They create all kinds of endorphins in the brain. Welcome to the world of knowing what it is like being a healthy fully functional male. Fortunately males have had since puberty to learn to cope. Fortunately sexually aggressive women are more excepted than sextually aggressive men.
Also steroids tend to make the clit a little more sensitive meaning something as simple as your panties rubbing up against it can create sensations that cause arousal much the same way a guy goes erect your clit will experience added blood flow.