Welcome to Just Answer. The reason is that no one here can examine you nor run blood nor urine tests to determine why your legs and feet are swelling. It is essential that you seek out a free clinic to keep a check on your diabetes. Numbers in the 300s are not good. Please use the links that the doctor sent you to the Lantus patient assistance program so you can get back on your basal insulin plus whatever NPH you need to take to get your glucose under control. Some of the glucose monitor makers also have patient assistance programs for those who do not have strips, and some doctors have free meters and strips for folks who can't afford them.
Walgreen's Take Care clinics are also beginning to offer free primary care during limited hours to a small segment of people who have lost their jobs and have no insurance. You can also call your local social services agencies to see if you are eligible for Medicaid or some other form of assistance. Until you can get an examination there is no way to tell what is causing the swelling in your legs. It can be problems with the valves in the veins, a kidney problem, heart failure, and several other things.