I have a small cyst in the southern part of my scrotal right where the sac meets the skin. it feels like it is on a vein. any ideas what this could be?

Needs exam: To give a good answer requires an exam. Possibilities include a varix (bulging of a vein), a small lymph node (if firm or fleshy), a cyst (if round and very firm/hard). Most such swellings if they are the only symptom will turn out not to be a serious problem.Get your doctor to examine it if it gets larger, tender, hurts, gets red, or drains pus.