Suggest Remedy For Sore Darkened Spot On My Sternum

Hello, I'm a 17 year old young, African American woman. In January, I notied a darkened spot on my sternum (I'm pretty light) and as time went on, that larger the spot got. It's April and it's still there however, now it's began to swell as well as hurt when it's touched. I'm very concerned and not sure what to do.