Will a penis shrink from protracted none use?
Male/female menopause. My wife of 32 years and I noticed that my penis has become smaller, her areolas have become smaller and more pale. All part of the aging process as nature tells us we need not reproduce any more.
(But just in case it's hormones you're taking, check with the doc.)
I'm not sure what could cause that.
Anabolic steroids (not the ones usually prescribed by a doctor) are linked to testicle shrinkage, but there is no evidence that they would reduce penis size.
Other types of steroids (more than likely the ones you are on), are generally not related to this side effect, as they are not designed to increase testosterone.
If that was your worry, that wouldn't be the cause.
Besides that though, I'm not very sure. You might want to mention it on your next doctor's visit though, and just see if he can give any more insight.