Hello and welcome to just answer. I am pleased to assist you and do not hesitate to ask follow up questions. It sounde like that buldge is part of the venous system that fills up during erection. Once flaccid, the blood is no longer pooled there and you are then unable to feel the buldge. You may be able to flatten it and watch it refill with blood again.
If it is very hard or becomes painful, then please see your doctor promptly. It does sound to be like a vein...part of the erection. Diagram at http://www.asiaandro.com/archive/1008-682X/4/61f1.jpg
I do hope I have helped. It has been my pleasure assisting you.
Cartilage like is fine as this is probably similar consistancy the the rest of your erection. The underside (corporus cavernosa) does tend to get particularly engorged. It was likely there for a while but smaller. If it continues to grow or becomes bothersome, a urologist will be able to assist you further. Certainly bring it up at your next doctor's appointment where your doc can take a look at it if needed.
I hope this clarifies things a bit more. Is there anything else I can do for you?