Multiple issues. Penile length when flaccid does not change with age unless there is fibrosis to the corpora (erectile bodies of the penis). This happens with injury, infection and some other systemic illnesses. Weight gain in the suprapubic area can give the appearance of shortening. Having had your prostate removed (prostatectomy) can also cause penile shortening.
Usually not. Penile shrinkage occurs due to decreased blood flow to the penis, penile plaques that cause scarring and curvature of the penis, or prior surgery that causes one the two previously mentioned problems. An exam and interview should help determine what happened and how to treat your problem.