It does not sound like a keloid to me. Keloids develop over time, and from what you said it sounds like this appeared quite quickly. It sounds to me like what is called 'Hypertrophic Scarring'. This type of scarring is reversible (you can get rid of it). It is caused by excessive irritation to the piercing. This irritation can come from overcleaning, overusing salt water soaks, playing with the piercing, changing the jewelry too often, etc.
What you have to do to get rid of it is quite simple : take care of the piercing properly. Clean the piercing once a day with antibacterial (Dial) soap; not more than once a day. Overcleaning causes severe irritation which can include dryness, itchiness, flakiness, etc. Overcleaning also prolongs the healing process of the piercing, which you do not want to do since navel piercings take a year to fully heal.
Salt water soaks are a major problem especially since most people do not know how to use them. Overusing and abusing salt water can be detrimental to your piercing. I personally do not use, and do not recommend anyone else to use salt water. Overusing salt water can damage the tissues in your piercing and around it, causing dryness, cracking, peeling, etc. Also, overusing salt water does not allow the piercing to heal... period. It will never heal as long as salt water is abused. Salt water also does nothing for healing. It has absolutely no healing qualities for your piercing. In the best case scenario, it will do nothing for the piercing. Worst case scenario, it could damage your piercing so badly you will have to take it out, and you will have scarring. If you would still like to use salt water soaks (some people still do for peace of mind), you will find information on how to properly use the soaks in this sticky : http://www.askmehelpdesk.com/body-ar...rt-195834.html
Remember, do not use the soaks more than once a day. It could damage your piercing.
Do not touch your piercing unless it is to clean it. Don't twist it, tug it... nothing like that. Piercings are meant to be ornamental for your body, they are not meant to be a toy to play with. Make sure that when you touch it to clean it, you always wash your hands first. Dirty hands spread bacteria.
Pick one piece of jewelry, and stick with it. Don't change the jewelry over and over again, because the piercing will get irritated. Don't use flashy or dangly jewelry, this kind of jewelry is quick to get caught on things and tear your piercing.
Wear long shirts and loose jeans, skirts and shorts. Any kind of pressure that is put on the area is only going to irritate it further. Avoid bending over for long periods as much as you can.
If you eliminate the irritation, the bump will go away. Just be sure to properly care for it and you will see results.