What Does Growth On Top Of Belly Button Indicate?

my sister had a growth just right on top of her belly button, it has been 2 weeks since the pain. there's no sign of redish/swelling from the outside. was 0.5cm last tested, and now she feels its about 2cm. the shape of it is like a crescent. she felt the pain in her stomach when she walk.

doc do not know what that is, am afraid it's a tumor or something...

kindly advise.

thanks.