Creaping black spots

[melnkai]

Hi, can anybody help? I have creaping black spots in the bottom of my bath tub and dont know why they have appeared. I hav'nt done anything different or used new cleaning products. I have tried cleaning and bleach but the spots seem to be under a layer of the plastic bath, they range in size from 1/2 inch to 1 inch and a creaping on a daily basis.

Does anyone know what this is caused by and is there a way to resolve it?

thankyou