Re: Decrease in size of my penis.
Do you have any pain? Is it possible that you injured your penis? Has the shape changed at all? Has it changed direction? Is there ever pain with urination or a discharge? (You don't need to have ever had sex to develop a urinary tract infection.) You aren't on any medications, are you? Might you have high blood pressure? Or low?
Are your erections softer than they were before? If so, I might wonder about psychological issues, physical obstructions and/or hormone levels. Are you under a lot of stress? Are you worried about things? Do you ever have any pain in your testicles? Do you ever have pain that feels like it is coming from behind your penis? Besides low androgens (male hormones), high levels of other hormones can interfere with androgens (estrogen/prolactin/etc.).
Do you ride a bicycle? Bicycle seats can cause many erectile problems for men. Get a wider/flat seat if you must ride a bicycle. Regular bicycle seats press on the perineum too hard.
Are your erections still very firm? This is probably not likely, but could it be that your foreskin has become tight? This can force the shaft down into the pelvis rather than letting it fully expand outside the body. I think 2" would be a pretty astronomical loss of this sort, though.
That is all I can think of, but there are tons of things I've never heard of. No one here is a doctor. Chances are good that a doctor could identify your issue right away with a physical exam.