What Causes Spreading Bump Under Breast?

I am 53 years old. For as long as i can remember i have had a small hard bump under my left breast. When I was pregnant with my first child i asked a doctor about it and he said it was just a pimple under the skin. Well it has never gone away. It has gotten larger over the years. At times i have to lift my breast up off of it because it hurts. This has gotten more frequent. In the last week the bump or knot has gotten a lot larger and red and appears to be spreading and getting larger quickly. Is it still just a pimple under the skin that is staying there or a fatty knot that is irritated or........