My Penis Has A Distinct Constriction On The Left Hand

My penis has a distinct constriction on the left hand side, about half way down the shaft, but only when fully erect. It is not Pyronies disease because it is not bending. From what I have read, constriction of the penile shaft usually is related to using sex toys, but I do not. In fact, I rarely have had much sex over the last 10 years. Currently no pain, but it seems to be constricting more over time. It occurred over the last 6 months to a year. Prior to that it looked completely normal. It is not painful, either flaccid or erect. I can urinate without pain and ejaculate without pain. But it looks funny and seems to be slowly progressing. When flaccid, I feel no scar tissue where the erect penis is constricted after several thorough self-examinations. I am afraid it will progress and cause me future difficulties. What could be causing this. I assume I need to see a Urologist.