What Does Discolouration Of A Mole Indicate?

I have had a mole on the back of my leg for years. I'm 34, fair skinned, and have many moles and freckles. This one is usually red and slightly raised, but off and on it will turn black for a period of time, and then it will go back to red. I thought it was a cherry angioma. I had my Dr. look at it about a year back. She said probably nothing but keep an eye on it. Last week, it felt irritated but was still red. Now it is black and elevated and sore, with a bit of irritated, red skin around it. I see another Dr. in 2 weeks and will have it looked at. I'm worried that it has been cancerous for years, and that I'm now in big trouble. Can cherry angiomas change like this and not be serious? I'm very scared.