This sounds to be the follow-up after priapism. If you had a prolonged erection (usually of several hours duration), then there may have been some inflammatory changes in your penis that you feel as the persistent firmness. You might consider an antinflammatory medication such as ibuprofen (if you can tolerate the medication) and I would avoid erections until this problem resolves and ceases being uncomfortable.
If the problem persist more than a few weeks, see a urologist.
I have not heard of L-arginine being of any benefit.
S.A.Liroff
I don't know if it is the same as having injections to combat ED. If it is then a visit with a doctor is needed. An erection lasting more then 2 hours without the penis returning to it's falcid state does cause damage.
God bless,
Ron