Does a navel piercing causes you to have s cyst, weight gain?

Does not explain weight gain: I am assuming when you say you have a cyst, that the cyst is around the area of the piercing. Not having the advantage of examining you i don't know if this is infected or not. I recommend that you remove the belly ring immediately, and gently cleanse the area around the navel. If the skin is red or if there is any discharge or blood from the area, i urge you to seek medical attention right away.

Yes and no: Umbilical piercings can develop cysts, which are easily treatable. They have no significant impact on weight.

Has the piercing: fully healed? Do you have signs of infection? Where is the possible cyst? Weight gain - no.