I (and others) have reported swelling after strenuous use of one's penis (i.e. aggressive masturbation, pumping, sex, etc.). For me, it's never been painful, and it's remarkable how much thicker I get for a day or two after the fact. I used to worry about it but now it just comes with the territory and doesn't appear to have affected any form or function of my unit.
I'm sure this reaction varies person-to-person, but I haven't experienced or seen anything to warrant concern. I would be curious to know the mechanism behind it, and if it could potentially pose an issue down the road; however I'm 10 years post-op and have yet to experience a single complication from my injections.