Normal variant. Relax
I went through the query and understood your concern.
Since the swelling is present during the time of masturbation
, the cause may be due to pooling of venous blood
which is quite common and will be back to normal.
Moreover you don't have any symptoms like pain or discomfort which suggest the normal variations. So no need to worry and be relaxed.Quit masturbation for a week and you will experience/observe the same status as before.
If the swelling increases or any discomfort then doppler scanning will help in diagnosis if any pathology. So till then don't worry but needs observation.
I hope this information helps you and answered your questions.
Feel free to ask for any further queries. I will be glad to answer them.
Wish you good health.