Hi, I'm bianca
Since I was born, I have a black beauty spot on my backside and ever since, it started to grow rapidly. It is the same size as my thumb. What should I do ?

I seriously think you need to get that checked out, just for safety precautions. Things like that go unnoticed and uncared for until it's too late because it develops into skin cancer melanoma. The doctor will address this specifically.

You say you have a big beauty spot on your back since you were born - do you mean a birth mark ?
If this is the case then yes it will grow as you grow.I have one on my tummy which has grown as I have grown,and is now quite large but in comparison to the size of me.

However if it is a spot that is not a birth mark and has suddenly started to grow in size, then I would pay a visit to your local GP ,who will be able to tell you exactly what it might be and how to treat it.