Is my belly on infected?

I got my belly on piercing abouts 4 weeks ago, and it has been fine but ever since a couple days ago its started to hurt, and whenever i sit down it really hurts i don't know if its infected or not and sometimes this yellowish greenish looking puss comes out? if it's infected what should i do? i know im going to keep it in but will it heal if i keep it in and will it get better? is there anything i can do