Thank you for your advise. I will contact my doctor ASAP. Unfortunately this is a holiday season, so this may take time, especially when you have to go through secretary, voicemail system, and you can not contact your doctor directly.
To Moderator:
Having painful erections (priapism) maybe for the rest of my life is not what I am hoping for. I think, the mechanism of these erections is as follow (of coarse this is just my speculation):
1. The nerve bundles, responsible for erection, are attached to the prostate on purpose - when prostate is enlarged, it is pressuring against these nerves, keeping penis erected (that's why in puberty when prostate grows, involuntary erections are normal). The other nerves, responsible for orgasm, are not near prostate.
After ejaculation, prostate size decreases, thus reducing pressure on nerve bundles and causing penis to become flaccid. Next erection is not possible until prostate is filled with liquid again.
2. I think something is pressuring on my nerve bundle (even if it is cut it is still alive) and sending wrong information to my penis to erect. From the fact that this happens after exercise (like walking stairs), I conclude that internal pressure increases, causing this problem. The question, which may be difficult to answer, is: "will this last forever, or is this caused by healing process only?"
And why do you think this may be a good news? Give me some hope.