No, don't think cancer. Have you had sex prior to this? Friction can cause this. The swelling makes it puff up, hurt and the grey is just because it is all puffed up. If you haven't had any type of friction there, it could be a skin infection called cellulitis. If that is the case, you have to see a doctor for antibiotics. It's been a few days. What is going on now? Is it any better?