Hello and welcome to the forum. Sorry for the delay in response to your question. Sorry for this situation. It is hard to say if your rubbing of the bump has caused hyperpigmentation or if the pigmentation has happened for another reason. Because of that, it would be best for you to see a doctor to evaluate that. I will say though that your rubbing could damage the skin to some extent and this is one of the listed causes of hyper pigmentation. There are home remedies that have been used with varying degrees of success on dark areas of the skin such as dabbing with lemon juice on a cotton swab, using a lightening cream, making a mask of honey and brown sugar, trying tea tree extract on it. A dermatologist will have other things to try including laser therapy which is known to solve the issue for many. But hopefully it will fade on its own. It's good it is not always visible based on lighting. I'd recommend sunscreen always as you are apparently prone to hyper pigmentation. Again, it is always best to work with your doctor on this.