My first question is why haven't u gone to see your doctor??????? U could have a blood clot! I would run not walk to the Dr. and your diabetes, ask yourself this if in the last yr it has gotten out of control, why? Maybe u have stopped eating the right foods or maybe u need medications to help u now. Run, run, run, to the doctor....
Have you seen any doctors lately for your pain? Sharp pains down the back of the leg can be a number of things; Sciatic nerve pinch, peripheral artery disease like Atherosclerosis (hardening of the arteries), abnormality in a disk or the spinal canal—a tumor, an infection, or the bone itself - Osteomyelitis.
With a 20 year history of diabetes, complications may arise which may or can lead to amputation. Go see a doctor asap and get to the root cause of your pain before its to late. Also, try posting on the Ask the Doctor forum - see the link below from the Med Help post on 10/03
It could be nerve related especially if your blood sugar is not under control. Hopefully you can see your dr and change your medications to help you. There is a medication called Lyrica that can help nerve pain. You have to start taking it gradually, and increase dosage slowly. Doctors will usually start you out with samples to see if it will help you. It can cause you to gain weight tho. I had to stop taking it because of that, as I need to lose weight not gain it. Good luck, and ask your dr.